I finally found a way to calculate the Minion life cap. And even created an excel table (open office) for that purpose. You can go ahead and download it right now from my attachment or you can let me explain first what the table actually shows.
First of all: I tested out the minion life of every necromancer minion by unequipping all charms and all of my gear and thus using the base values of the minion HP at skilllevel 1. Luckily I found a T5
Fauztin's Visage
and got 25% minion life on it, so it was perfect for testing purposes.Fauztin's Visage
Field Plate (4)
Defense: 268 to 540
Required Level: 11
Required Strength: 32
Item Level: 19
+(11 to 20)% to Summoned Minion Life
+(6 to 10)% to Summoned Minion Resistances
+50% Enhanced Defense
+250 Defense vs. Melee
+50 to Life
Poison Length Reduced by (21 to 25)%
Physical Resist 1%
Socketed (2)
Defense: 890 to 1668
Required Level: 37
Required Strength: 63
Item Level: 36
+(21 to 30)% to Summoned Minion Life
+(11 to 15)% to Summoned Minion Resistances
+100% Enhanced Defense
+500 Defense vs. Melee
+100 to Life
Poison Length Reduced by (31 to 35)%
Physical Resist 2%
Socketed (4)
Defense: 2232 to 4107
Required Level: 61
Required Strength: 219
Item Level: 59
+(31 to 40)% to Summoned Minion Life
+(16 to 20)% to Summoned Minion Resistances
+150% Enhanced Defense
+750 Defense vs. Melee
+175 to Life
Poison Length Reduced by (41 to 45)%
Physical Resist 3%
Socketed (5)
Defense: 3642 to 6678
Required Level: 75
Required Strength: 438
Item Level: 85
+(41 to 50)% to Summoned Minion Life
+(21 to 25)% to Summoned Minion Resistances
+200% Enhanced Defense
+1000 Defense vs. Melee
+250 to Life
Poison Length Reduced by (46 to 50)%
Physical Resist 5%
Socketed (6)
Field Plate (4)
Defense: 268 to 540
Required Level: 11
Required Strength: 32
Item Level: 19
+(11 to 20)% to Summoned Minion Life
+(6 to 10)% to Summoned Minion Resistances
+50% Enhanced Defense
+250 Defense vs. Melee
+50 to Life
Poison Length Reduced by (21 to 25)%
Physical Resist 1%
Socketed (2)
Defense: 890 to 1668
Required Level: 37
Required Strength: 63
Item Level: 36
+(21 to 30)% to Summoned Minion Life
+(11 to 15)% to Summoned Minion Resistances
+100% Enhanced Defense
+500 Defense vs. Melee
+100 to Life
Poison Length Reduced by (31 to 35)%
Physical Resist 2%
Socketed (4)
Defense: 2232 to 4107
Required Level: 61
Required Strength: 219
Item Level: 59
+(31 to 40)% to Summoned Minion Life
+(16 to 20)% to Summoned Minion Resistances
+150% Enhanced Defense
+750 Defense vs. Melee
+175 to Life
Poison Length Reduced by (41 to 45)%
Physical Resist 3%
Socketed (5)
Defense: 3642 to 6678
Required Level: 75
Required Strength: 438
Item Level: 85
+(41 to 50)% to Summoned Minion Life
+(21 to 25)% to Summoned Minion Resistances
+200% Enhanced Defense
+1000 Defense vs. Melee
+250 to Life
Poison Length Reduced by (46 to 50)%
Physical Resist 5%
Socketed (6)
I wrote in the table the Base life without any +minion hp bonus, then I added the 25% minion life and wrote down the value that is shown in the game. Afterwards I calculated the Life that the skill should show. Afterwards I took both the the shown life and the calculated life, substracted the the base life from both and then devided the shown life and the calculated life.
So:
Multiplier = (Shown life - base life) /(calculated life - base life)Since this is very inaccurate at low values I tried it out with 3 different + minion hp percentages to get the best and most accurate result.
What's very interesting is the fact that different summons have different Multipliers. For example Lamias and Rampagors have the same multiplier, but Skeletal Flayers have a different multiplier.
Afterwards I used this multiplier to find out the base HP (for each minion it's seperate) which I needed in order to find out how much life your summons actually have.
Base HP= shown HP/(minion life bonus in % x Multiplier)Now in order to find out the real HP value of your minions if your skeletal flayer Aura is active we add 50% of the base HP of the minions to your minion HP to find out the actual HP your minions have while skeletal Flayer aura is active.
Minion HP with SF aura= Your minion HP /(Your minion HP % bonus*0,5)Those are the formulas I used. Please be aware that I used the multiplier values from the 3rd round with 316% minion life and +0 to any skills in order to have the most accurate results.
So what do you need in order to calculate it on your own special case?
Just download the file, and enter in the correct column (the names of the minions are at the top) the life that is shown, and the bonus to minion life of ALL your items. Make sure to add the bonus of "Embalming" to it as well. So if your equipment has + 500% minion life and Embalming has + 100%, you actually have + 600% minion life.
Then you can find out how much life your rampagors for example truely have if you used Skeletal Flayers
So why should you use this calculation?
easy. You can simply add the %minion life of jewels to the calculations before equiping it. The only requirement is to have the same +skills all the time though since +skills change the base HP.
But how did I find out the cap is 90k?
I tested it out. I used a different staff and more +minion life jewels. That way I got to 997% minion life and my rampagors had 88672 life. They died in act 2 rocky waste 1 hit to any attack. The amount of life shown in my table is 90463 if you add those 2 values. Enemies in destruction do at least 500 damage as explained here. That means the cap is 90 k or at least very close to it.
Well, that's as far as my calculations can go.
It took me 2 days to create this easy thing (due to calculation errors
) so you can at least give me an upvote if this was helpful to you 
Thank you for your attention and I hope devs can confirm this.
