mileage expenses form formulas

Hi everyone.
I have created a mileage expenses form in which the start mileage is placed in say column H, the end mileage in column I, and the actual mileage done in column J
I have always used a simple manual calculation to give the mileage figure by subtracting say H3 from I4 but is there anyway i can do this automatically, bearing in mind there is usually a different number of visits each day therefore the number of rows are not constant between days.