Form not calculating correctly

Hi,
I'm trying to create a form for Mileage Reimbursement here at my company. Most of it works fine, but when I try to calculate the amount that should be reimbursed by multiplying the rate by the number of miles, the calculation occurs a step behind. So, I have several boxes Miles.0-Miles.5, then a field Amount that adds up the Miles. That works fine. Now I have to multiple the Amount by 0.51, so I created a box that has a default value of 0.51 and is read-only. Then, I have a final box called Text3 that multiples the 0.51 box by Amount. The multiplication itself is fine, but it occurs a step behind! I'll type in the first mileage, but no calculation will occur. Then I'll type in the second, and THEN the form calculates that first mile I entered. What could be causing this?