Anyone used the Helmholtz equation to determine sound hole size?

Here's the Helmholtz equation I've gotten:

(F)(F) x Volume x 0.00000023895

where F is the frequency (Hz) of the 2nd lowest string.

Am I correct to say that in DAD tuning, the A is 440 Hz?

Using this formula, it seems that most sound holes I've seen in standard mountain dulcimers are way too small for the volume.

That's why I'm asking if anyone else has used this equation? Just not sure it is correct.