If you look in the Variants folder there are a bunch of variant files for Magic Spells based on different attributes. Looking them over quickly there's one called
Variant Magic 4e - Alt Attribute (IQ + Will) ÷ 2.gdf which comes close to what you want. So we'll open that file up and right after the copyright blurb there's the following code section:
Code:
************************************************************************************************************************
[SKILLTYPES]
************************************************************************************************************************
SP/H, cost(1/2/4/8), base(-3), defaultstat(@int((ST:IQ+ST:Will)/2)), relname(IQ+Will/2)
SP/VH, cost(1/2/4/8), base(-4), defaultstat(@int((ST:IQ+ST:Will)/2)), relname(IQ+Will/2)
So this defines two new skill types, SP/H and SP/VH. It bases them off of IQ + Will /2, so all we need to do is swap out Quintessence for IQ in this code to change it around:
Code:
************************************************************************************************************************
[SKILLTYPES]
************************************************************************************************************************
QT/H, cost(1/2/4/8), base(-3), defaultstat(@int((ST:Quintessence+ST:Will)/2)), relname(QT+Will/2)
QT/VH, cost(1/2/4/8), base(-4), defaultstat(@int((ST:Quintessence+ST:Will)/2)), relname(QT+Will/2)
Et voila, two new skill types defined. Since spells don't (or didn't) have easy or average skill types it didn't bother to define them. However, the only real difference is the base() penalty, so SP/A would have base(-2) and SP/E would have base(-1)
Code:
************************************************************************************************************************
[SKILLTYPES]
************************************************************************************************************************
QT/E, cost(1/2/4/8), base(-1), defaultstat(@int((ST:Quintessence+ST:Will)/2)), relname(QT+Will/2)
QT/A, cost(1/2/4/8), base(-2), defaultstat(@int((ST:Quintessence+ST:Will)/2)), relname(QT+Will/2)
QT/H, cost(1/2/4/8), base(-3), defaultstat(@int((ST:Quintessence+ST:Will)/2)), relname(QT+Will/2)
QT/VH, cost(1/2/4/8), base(-4), defaultstat(@int((ST:Quintessence+ST:Will)/2)), relname(QT+Will/2)
Now all you have to do is give your new skills the type QT/H or whatever to get it to be based on QT+Will/2. And, remember, on any existing character you are going to need to to a Resynchronize to get the new skill types recognized.