How much core strength is enough?

I'm targeting achieving movements like a handstand, human flag, front and back levers. Currently I'm unsure if my core strength is enough to perform these movements. I can hold a plank with 45Kg additional weight for about 90 seconds (Is this a good way to measure core strength?). Should I start training specifically for these movements or train core some more? Please do suggest good hardcore (pun intended) routines.