field A cannot be more than 50% of field B

I have a form which has two number fields that house formulas.  However, the second field can not be more than 50% of the first field, no matter what the calculation totals.  I.E.....

field 1 + field 2 = field A ($500)

field 3 + field 4 = field B ($400)

But field B cannot be any more that $250 (which is 50% of field A)

How can I do this?