Helmholtz resonator equation help? Anyone?

Don't know if this forum is the right place for this question, as I'm trying to figure out how to build a bass trap with this formula. But thought I'd try.
Anyways, does anyone know or know how to work the helmholtz resonator formula, for constructing a perforated box?
I seem to be getting conflicting results with my mathematical formula proceedures for some reason. Don't know why. Also, does anyone know where I can find a "plug in" or whatever to simplify this process? I'm a bit "puter ill-literate", sadly.

I have the following formula, but am getting conflicting "right answers" when I try to work it. Here's what I'm using from F.Alton Everest's Master Acoustics Handbook:

F = (200)x square root of (P/(D)x(t))

In which F = target frequency,hz
P = perforation percentage
t = effective hole lenght, inches, with corretion
applied
= (pannel thickness) + (.8)x(hole diameter)
D = depth of air space, inches.

Now, this same book says that if you use common peg board, with a .18 inch (3/16ths) thickness, and same hole diameter, with 1" spaced holes on center, at a depth of 4"(er, 3.625" on 2x4 studs), your reasonant fequency is around 420hz. I seem to get anything but that when I calculate!
Maybe someone has some input here? I know it's an "off the beatten path" post at that.
Any input will be greatly appreciated!
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