pathfinder homebrew:need help with arcane dc

[Redhat Mike]

Hello this is my first time posting but long time listener.I am working on a long running adventure I have plan to start in January or possibly sooner with my friends.This world I am making is setup where arcane spellcasters after they use a particular spell and they can attept to focus their will/powers into reactivating that particular power.I will not have no problems with how many times they want to do this but each time they attempt to do this I need the dc to go up.The problem I have is what would in your guys opinion would be a good base for the dc to start and progress? (So you guys know should they fail the dc the spell would bouce back at them.)

[Yora]

That depends a lot on what they would be rolling to reach the target number.

An important thing about all d20 games is, that there are often very many ways to get a specific number very high. If one player would really want to, he could find ways to make sure he will reach the DC almost every time.
That usually requires some creative work by someone who knows the rules very well and puts some effort into it, so it's not very likely to happen, but you should probably still base it on something that is not easily increased. Like a caster level check (1d20 + caster level), which are almost impossible to increase in other ways than gaining new levels.