Ok here is the data I have:

Invoice Amount (total with everything together) $9.12
Sales Amount ( 1 part ) $4.49
Tax AMount (tax changed on part + shipping cost) $.68
Shipping and handling $3.95

In textbox1 i put the tax amount
In textbox2 i put the invoice amount

then it displays the tax percentage in textbox3

then i put in the part price in textbox4 and it * by whatever is in textbox3
and puts that into textbox5

So the way i double check to make sure its right is, i add shipping and handling + sales amount and then * it by percentage, and it should equal out to the tax amount.