Formula for Excel

I work for a Financial Advisor company & am setting up a excel spreadsheet to automatically calculate monthly payments for mortgages.

One question:

In an already formulaed cell - can i have two alternative calculations with someway to switching from whatever calculation is required

e.g.

in cell B6 i have the loan to value formula which calculates the mortgage amount (B5) against the valuation of the property (B4) i.e. =SUM(B5/B4)

what i want it to alternatively show is the max loan available for a particular product i.e. =SUM(B6*B4)

can this be done

kind regards