I can't see how you can mess up the ignition timing on that engine Mark - if you've found a way I'd be interested. It seems to me that if you put the flywheel on the key, and bolt on the ignition unit in the only place it will fit, the timing is correct.
There is a reason to have the TDC mark. Because Honda does not use a timing mark on the crankshaft timing gear, to reassemble the engine without making a timing mark, you have to put the engine on TDC, then slide the camshaft timing gear into position so that its timing mark is at the closest possible point to the hub of the crankshaft timing gear. Because the timing gears are helical, of course you have to lead it a bit so that by the time it rotates as it slides down into place, it is as close as possible to the crankshaft. Kind of reminds me of duck shooting.
