Hi,
I would be inclined to do this using VBA code or a query, however, the problem might be the 2 input parameter fields, being imprecise.
What I mean here is if the user inputs 5' 9 1/2" as a height and a circumference of 42" (I assume waistline)

,and if you don't have anything in your lookup table that satisfies these parameters, then you are going to need to search for the closest Body Fat % fit.
Please attach the DB to this forum and I'm sure we can give you some pointers.
Kimbo