Formula problem

[David]

I would be very grateful if someone could help me with the following formula
problem i have cell H4=((PI()*(E4))/(6.5*F4))
What i would like to do is have a formula which is calculated when there is
either a value in F4 or G4. However I don't want G4 multiplied by 6.5.

 

[Toppers]

this ..?

=IF(F4<>"",PI()*E4/(6.5*F4),PI()*E4/G4)