Andy,

You will know the TDC for combustion because that is the stroke that will have compression. The other TDC will not have compression. When I did my engine, the little dot on the camshaft came in at 6 o'clock rather than the 12 o'clock as this picture shows from W8YE. In either case the dot has to be 1 tooth before 6 or 12 o'clock, which also happens to fall on the imaginary red line shown in the picture from W8YE.

It actually turned out to be easier than I expected.