HI Mguar,
What i meant with "not really possible" is " not realy possible without adding a timing scale". SHould have been clearer. Of-course if you add a timeing scale like you suggest it will be possible to set the timing. I already bought an orignal Jaguar V12 timing scale, just need to install it
I like your tips on finding TDC, but i don't see why you would need to remove the valve covers. On other cars, i just turned the crankshaft, looked at the position of the rotor in the distributor to align with #1 cylinder and used a rod like you suggest. I know that on the V12 you should never rotate the engine "the wrong way" to avoid issues with timing chain damper. But this will just mean it will take longer

 

You’ll want top dead center on the engine for firing reason. Since it’s a 4 stroke engine the first stroke is down drawing in air and fuel( intake valve open)
2nd stroke is piston coming up. Both valves closed. Top dead center of that stroke is where the ignition is firing( well, slightly ahead of TDC)

You don’t want to use the original timing marks because they are on the bottom of the engine. You can’t see them when you are up on top adjusting the distributor. Yes, you can lay on the floor and look up to see the marks but chances are you’ll use a normal timing light and the leads won’t reach.
yes you could make a temporary really long 1A lead. If you really like crawling around on your belly.

 

Non Jaguar trained mechanics fail to understand the overheating problem is often not anything to do with the radiator etc.
It easily can be the ignition timing, because it’s a V12 the engine will sit and idle smoothly and seem to run fine. Checking the timing on a Jaguar V12 is very difficult and awkward.
It could be the two radiator cape. Or the wrong thermostat. Etc.