RedRich,
I've been doing 21mm for a few years now and although the stores don't carry a check rail gauge (bizarre) but it's not absolutely essential. It sounds like a crude way of doing it but since it's the very last piece to go into a point I run a loco or a few wagons over the crossover after securing the check rail in place with very slow drying epoxy. This allows for any minor corrections of discrepancies to be taken care of. Technically the flange way at the crossing vee and the distance between the check rail and adjacent stock rail should be identical however I always go a little on the less tighter side with the check rail so there would definitely be no binding when the wheels pass through. I also use a single pair of wheels on an axle and place them on the track at the check rail that still isn't fully cured and gently move them perpendicular to the rails. I allow the flange (back side) of the wheel to just touch the check rail and then while rolling it back and forth check the opposite wheel to make sure it's not going to catch the crossing vee. This will allow enough play for the wheels to run freely with no binding but at the same time not too much that it will result in catching the crossing vee. Clear as mud? Actually, if you build the point well, all your rolling stock should roll right through without even needing check rails. In fact I've waited months sometimes before even installing them and have therefore come to the conclusion that for modeling purposes they are merely aesthetic.