I've made an excel sheet to calculate my handicap just for information purpose, I can share with you if you want.
The way i'm working is addind a row at the top when I play a new game and the formula should follow without any modification from your part.
The calculation is made by formula i've found on the web to calculate my handicap, I hope it's the right way to calculate it:
Differential = 113 * ( score - evalutation) / slope
Handicap = average of 10 best differential of the last 20 * 0.96
Also it's in french but it's pretty straight forward.
The handicap is at the bottom of the table.
The number on the right is just my average for the year.