For this example, I did the following:
A1: 0.61 (name this cell valC)
A2: 4.91 (name this cell valA)
A3: 3 (name this cell valH)
A4: 41.63101 (name this cell valQ)

Then put this formula in cell C1:
=IF(valH="",IF(valQ="","Insufficient
Data",(valQ/(8.025*valC*valA))^2),8.025*valA*valC*valH^0.5)
Now:
if you erase valH, the formula calculates valH.
If you set valH to 3 and erase valQ the formula calculates valQ.
If valH and valQ are blank, the formula returns "Insufficient Data"

I hope that works for you.

Regards,
Ron