To tune a horn/reflex comination first you measure for the Fs(h), with the driver in place but the rear chamber open to air. That Fs(h) is the lowest frequency you can tune the Fb to, and doing so gives you the lowest extension but also the least sensitivity at the horn Fc. To gain more sensitivity at Fc you can tune higher than Fs(h), with a loss of low end extension; where to go depends on what you want to get. I wanted 80 Hz so that's where I tuned to with DR200.

There comes a point where you can't tune any higher, so then you just go sealed. If that gives an Fb that's still unacceptably lower than the Fc then you have to reactance annul by making the sealed box smaller.

I measure with a sine wave generator as a source and a Phonic PAA2 doing the measuring; it is accurate and has an unweighted mic setting. I don't have software for my charts, I make them up in a Works spreadsheet.