I run the peacemaking mastery and I will cast rising colosus when running the roof. I do get into mana issues. My LMC is 39/40 and my mana regen is 25/30. What should be my goal?
I'll start by saying that using double bard mastery AND casting spells is likely going to lead to mana issues. My bard runs 120 med (also has like 40 MR) and for the events that I use him for support but also cast spells (heal/cure/etc), I will also start running low on mana if I don't watch what I'm casting. I think that's just the nature of things.
Going to make a few assumptions here.
I'm assuming since you are casting RC that you have 120 focus?? Also assuming if you have 120 focus you don't have med. Last assumption is the armor you are using is mage armor.
My advice on this would be to remove the mage armor property / swap over to gear with the +3 inherent LMC. Goal is to be as close to 55LMC as possible. From everything I've read on MR - about 30 is the point of diminishing returns so aim to be at 30. I'm not sure what you are using in your hand but the shogun wand from the last event has +10 MR (and 0 mage weap etc) so that's an easy addition for a bard to instantly get a bump to MR if you aren't already using it.
When I start getting into a bit of a mana crunch, I will usually just turn off one of the two masteries I'm running and within a minute or 2 I'm back to full mana.