Hmm, that's about all you need to know about setting the basic timing.
The idea of retard is quite simple: the timing advance has to be lower on boost as it is on the idle. The theory behind this is a bit complicated, but you do not need to know much about it. If you're getting the BTM, it will do the job for you! That's why you need to get rid of the mechanical retard built into the dashpot on the distributor. It comes of by loosening two screws on it. There's an axle connecting it to the internals of the distributor, be gentle when pulling it out! Now the function of the mechanical retard should be very simple as you have the part in your hands: the pressure applied into the dashpot will push out the axle. And vacuum will do just the opposite. You do not want to touch the vacuum operation, only to defeat the retard. The way to do this is to look at the "knob" welded on the base of the dashpot. This is what prevents the axle from extending all the way out as the pressure is applied. What you need to do is to bend that knob so that it will not allow the axle to extend. There is only about 2mm gap in stock form, you need to get rid of it. Just punch the knob with something so that it bends. Don't bend it too much, you do not want to have it pushing the axle into the "vacuum" area. That's all!
The best way to test that you really got rid of the retard is to first check your basic timing (above) and then apply pressure to the dashpot (i used a half-empty tire, a hand pump with a pressure tank would be even better) and check the timing again! No matter how much pressure you have in there, the timing should not have retarded a lot. I have one degree left in my unit and that's fine.
Now comes the btm: when you first install it leave the retard knob at zero. You do not want to run around with excess retard. Setting it properly is a timing light excercise as well! Hook up the light. Connect the pressure tank (or whatever youre using) into the hose going to the BTM box. Now what you really need to have is a pressure gauge somewhere in that hose. This is important because you need to know at which pressure you're at when checking the timing. Start by pumping about 15PSI into the tank. Now with the engine idling, measure the timing (this is really ackward if you don't have a timing light with a gauge). What you want to see is a timing of about 10 degrees at 15PSI. You need to experiment with the BTM pot to archieve this. The potentiometer should need to be turned only a few degrees up from the zero level, so be careful!
As soon as you got it to 10 degs at 15PSI it would be a good idea to measure the values from the whole pressure range. Start by noting the basic timing once again (just to make sure you got it right), this is without any pressure going to the BTM. Should still be 18 (or whatever you decided). Now pump some pressure into the BTM. Measuring the timing for every PSI added would be my recommendation: you can plot a curve which should be linear: this will confirm that you have not made mistakes in the measurements.
The last trick in the field of timing would be the pressure control valve (or bleed valve). This is not absolutely necessary, but if you're running with pressures much beyond 15PSI it is strongly recommended. The idea of the valve is to limit the pressure going to the BTM to a certain level. This way the BTM will retard the timing in a linear fashion until a certain level and then stop retarding, no matter what the actual boost is. This is useful because the ideal retard curve up to about 15PSI goes so that the timing at that point is about 10 degs BTDC. No matter how much you're boosting, you do not want to have much more retard than that. The pressure valve, installed in the hose going to the BTM, should be set so that it peaks the pressure at about 16PSI (my experience: again, you should experiment yourself). This way the BTM will retard the timing all the way to 9 degs BTDC. Now the actual boost in the engine may well be 20PSI but the timing will still be at 9 degs. This has worked well for most people, but again, you need to experiment yourself to find the best setting for your car.