The angle isn't as important as you think. The main thing is, when you are on your first tightening sequence, certain valves will be completely closed (your "sequence of valves"). After you turn the crank 240°, you'll do the rest of the valves. These valves need to be closed. On any engine I've dealt with, there's a pretty broad swath of crankshaft angle which will allow the 2nd set of valves to be closed. I don't know specifically for your engine, but it'll probably be +/- 20° at a minimum. Your best guestament at the 240° will put you close enough it won't make a difference.
An easy way to get it close is to put your turning device (breaker bar or what have you) at the 12 o'clock position (straight up), then turn it to the 8 o'clock position to get to the 2/3's point. As long as you are in the very close neighborhood, you shouldn't have an issue.
You could also put a mark at the 8 o'clock position from where the timing mark is at on the harmonic damper. Just looking at a balancer for your engine, it looks as though there's no marks on it. Instead, put a mark on the timing cover at the 12 o'clock position, then go 1/3 down the right side of the balancer and put a mark on it. When the marks align after turning the engine clockwise, you should have reached the 2/3's point and should be good. If you're careful, this should put you right where you need to be.