I think…
Each sheet has 4 columns (starting at A1) and titled:
Date
Mileage
Cumulative Milage
Allowance
Then below each heading enter:
Type in 1st of the month and drag down to the end of the month
enter daily milage
=IF(B2>0, SUM($B$2:B2), “”) and drag down to the end of the month
=IF(B2>0, IF(C2<100001, B2*0.45, B2 *0.25), “”) and drag down to the end of the month
Copy for each month
Sheet 13 is then just a sum of all monthly milages and allowances
Edit You could use a vlookup with a small table (4 cells) at the bottom of each sheet to make managing future changes easier!