On the Mk 12 Ignition timing is 3/16″ (4.76mm) before TDC, slacken the flywheel so that it just turns on the crankshaft, turn the engine to tdc, rotate it backwards until the piston is 3/16″ back down the bore (best to remove the head for accuracy).
Rotate the flywheel while holding the crank still until the points are just opening (use a cigarette paper, it should just pull out)
Still keeping the crank stationary carefully tighten the crankshaft nut. recheck the timing, if it’s OK do the crankshaft nut up very tight.
The Villiers spanner for this has a lump on the end which you hit with a hammer.