Updating cells to Formulas

Tankshop

I have a sheet that we track miles for vehicles for services so 1 column is for a A the next B service. those columns set of formatting against currant mileage and let the mechanic know if a service is due. the issue we have is the A is every 18K miles the B is 36K miles. When a B service gets done it resets the A service but i can not make this happen. I did automation so when they update the B mileage it will delete the A miles. Is it possible to make smart sheets update the A column to the B value automatically when adjusted but still be able to except the new A miles when that is performed. i tired o instead of clearing A making it say "Update" than another Automation for a formula to copy B to A column but it gives me an error also it states I can not add formulas in automation. Hopefully i explained this well enough