You have to remember that, when using Calculate, you're working with databuffers, not single cells like in GetDataCell. The dimensionality of databuffers has to match, so things like your first attempt are a non-starter: one buffer only has Account specified, and the other has Account and Time.
If you balance the buffer dimensionality, it should work:
api.Data.calculate("A#CF_RepaymentEquityInvestment = A#CF_RepaymentEquityInvestment:T#Pov + A#33100:T#PovPriorYear12")
If not, you should investigate what those buffers actually contain, i.e. if you're ever going to have values in the same intersections on both sides. Check out GetDataBufferUsingFormula and LogDataBuffer for that.