Fox 23, the best way to do it is to take the head off, then move the piston to BDC, then while the piston is there take a permanent marker tand mark the flywheel at the bottom. That way when your done running for the day you can easily just place the mark on the flywheel to the very bottom and not have to think twice about it. The other way is you can take the glow plug off and look down at do it that way, but it's a little harder to do it that way though.

Those are the best way's the other way is you can do it by feel, take the flywheel and play with it. Do a couple full revolution's. The piston while on the up stroke will cause you do actually have to use some force to get the flywheel to turn over. It will get real hard when the piston is near TDC, then once you get past TDC then piston because of the compression will want to move down by itself to a certain point. Then you will have to move the flywheel again, now the piston should be either at BDC or pretty close at this point. You will have to move it alittle further at that point. It should be easy to move around, and you will have about a(now this is a estimated guess here) about 30 degree's either way till you have to start to worry again.

Basically you want it as close to BDC as possible. The reason being is so the hot exhaust gasses can escape, and also the cylinder is tapered. The largest diameter of the cylinder is being at BDC, and the smallest diameter of the cylinder being at TDC. Now as you know when metal is hot it expand's when metal cool's down it will contract, well if the piston is at TDC the sleeve will not be able to contract as much and this will cause your engine to loose compression much faster then if it was placed at BDC. This is why we place it at BDC.

I hope this helped you more then it confused you. Maybe someone else will be able to better explain it.