>> "I assume I just reverse the formula to convert the other way? "

Basically Yes...

If you want to Save 27 minutes into the Minutes Register, then you save $27. The 2 is the twenty out of the twenty minutes and the "7" is the seven.

Remember, that when reading or writing you should never have an 'alphabetic' character... there are exceptions, some of the registers have the 7th bit as a special bit... just strip it out if it's not needed. The seconds register especially.... if the 7th bit is set to '1' then the clock isn't running - which is a way you can tell if the DS1307 is initialised or not.