With the back off the engine, I could see the crankshaft.  I mounted my degree wheel on the crank shaft.  I moved the crankshaft by eye to TDC by looking at it through the back of the engine.  I then moved the crankshaft 28 degrees BTC and marked the hub at the spot where I wanted the magnet to be.  I could have put the magnet and sensor anywhere on the case but I wanted it set at the very bottom of the engine.  The hub was marked where it matches up to where I wanted the sensor to go with the crankshaft rotated the 28 dgrees BTC.  Once again, I located tdc by looking at the crankshaft with the back plate off and the crank pin at the top of its travel.  You are right in that it is not the most accurate method and a small amount of movement can be several degrees.  That is why I learned that with this method it is necessary to fine tune afterwards when ready to start running the engine.  There is also a variance of when the spark occurs when the magnet passes by the sensor as well that has to be adjusted for.  I dont think it is possible to get it exact just from measuring because of the variances.

Jim

Dont worry about beating me up.  If you have a better idea I would like to hear it.  I am just a novice at this and am learning as I go along. </p>