In Ars Magica Casting a spell forcelessly is a deliberate act by the caster to inhibit their own spell power/spell strength so that they do not risk unintentionally cast a spell on another Magus. I’m not sure what a useful real life comparison might be – however it seems like the wizard is seeking to use enough of their knowledge and power to successfully cast the spell, but absolutely no more. Seems not without some risk.
Is that like quickly physically lifting an object up off the floor only one handspan? Or filling a bottle to the brim but not letting another drop spill? Do it fast but don’t go over.
Sounds like something which should require a basic Finesse check to do properly. Even rated as an easy task (say target value 3+) introduces the potential for mistakes and risk.
(As a counter – Ars Magica is a roll heavy game so perhaps KISS applies)
Perhaps if the Finesse check fails, the Casting Total is calculated normally minus the caster’s Penetration score. Sure, the magus didn’t mean to use a little extra force, but magic is fickle and sometimes hard to control.
On a botch (and this is a skill check and not a magical botch) perhaps the spell is cast as normal including the caster’s penetration skill. Whoops eh?