If you have the time and do the math on your own you may have a look into my Spell damage Guide.
The Synergy Bonus to Spell Damage that you can get from Energy Factor simply is capped. The formula is min[energy*EF/15000,200] so it depends on your Energy Value how much EF you need to reach the cap of 200 (@energy *EF =3.000.000). Take of the piece that you want to replace, look what Energy Value you have in buffed state. Add the boni from the jewels that you will use. Take that energy Value do 3.000.000/this value this is the amount of EF that you will need to cap the synergy.