If you make a rig, it needs to be made in such a way that, once slipped onto its axle the wheel will be dead vertical. Where you mount the axle onto the rig, the axle should sit on top of two, low friction bearings that are side by side. Two at one end of the axle and two on the other, so that the axle's only point of contact with the rig is the outer races of two parallel bearings either side, ensuring the wheel will spin very freely, making you balancing more accurate.

Once your wheel is ready to be re-tyred, then mount the wheel on the balancer, and spin it genly. Let it settle. Mark the bottom part (6 o'clock) and repeat. If the same point settles at the lowest point that is the heaviest part of you rim and wheel. The mark on the tyre is the lightest part of the tyre, and should be mounted next to that heavy spot. Basically you are offseting as much of the wheel imbalance to start with by using the imbalance in the tyre.

You could match it to the valve, but I prefer the method above. It's worth remembering that although the valve might be a bit heavier than other parts of the wheel, there was also a section of rim metal drilled out to allow for the valve, so the valve difference is not so great.

When balancing keep spinning the wheel gently till it stops and incrementally adding weights opposite the lowest point on the wheel, until you can spin it freely and your wheel doesn't really stop in the same spot over and over again. If it keeps stopping with a weighted area at the bottom (ie heaviest) then you could remove a single weight also.

Perhaps only add weight every other spin if you can see that it keeps stopping in the same position.
I find that correction fluid is a good marking agent to mark the rim as it spins. Its easy to clean off but visible on any wheel.

HTH

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