I have the great big hairy formula that takes TEMP and PERCENTAGE and delivers a precise SG. You could take that and solve for PERCENTAGE I guess. I don't have time to do it right now...sorry!
In spreadsheet speak with PERCENTAGE and TEMP substituted in for input cell numbers:
SG=((((100-(PERCENTAGE))*(0.792))+((PERCENTAGE)*1.139))/100)*POWER((528/(TEMP+460)),(1/3))
Spud