This post says that there is a one to one correspondence between Resource credits and SP.

Resource credits available seem to depend on three parameters: "blockchain size", "state size", and "computational load". The witnesses have an rc_api where they can see what these parameters are but unless you are running the witness code you can't see them.

I looked at several accounts on steemd and calculated the max_mana divided by the SP. The numbers all come out different. There doesn't seem to be a one-to-one correlation between mana and SP.