Lately I've gotten more PM's then usual in regards to tuning.
Here is the formula to be copied into MS Excel:
Fs=0.159*(((Av*(1.84*10^8))/((Vb*1728)*(Lv + 0.823*(Av^0.5))))^0.5)
Fs = Tuning Frequency
Av = Port Area
Vb = Enclosure Volume
Lv = Port Length
You need to make a few reference cells in the spreadsheet (whichever program). You'll need Gross volume, Net volume, Port area, and Port length seperate from this 'cell'.
This formula has been posted many, many times. As a matter of fact, that's how I came across it long ago. It meshes with a 1.5% +/- to BBP6 (when YOU account for port end correction).
Use it if you wish, if not, so be it. This has worked for me for quite sometime.