I am wondering if you get a dice roll with your parma when a spell is cast at you as it is an ability but i was told it is just the ability score*5 + form is that correct?
Right! But adding a die roll is not necessarily a bad houserule if you want some uncertainty to increase dramatic tension, particularly since some mystical effects (e,g. creature powers, powers of enchanted devices) are unrolled.
I think every group I've played with has used dice for Parma and etc. I'm not sure if it's a symmetry thing (spell gets a dice roll, so a creature power gets a dice roll, so magical defenses should too) or if it was actually canon back in 3rd ed. and I've carried it over to 4th all these years from habit.
There are stray references to resistance rolls in the core book for 5e, although they've been corrected through errata (see e.g. the special circumstances virtue).
Thanks for the help this cleared it up for me