SJB01, you're not alone in having difficulty judging the mast tilt angle, I think every FLT driver has the same problem because it's due to the fact that the driving position does not give any vision of the side of the mast or forks.
In training for smaller CB's I was told to lift the forks to eyelevel as this gives you the best chance of getting anywhere close to levelling the forks. However, it is still very inaccurate, does not work if your not on a perfectly level floor, and anyone who did this at work would be criticised for time wasting. And obviously it doesn't work at all when you have a load on the forks.
The best solution is to have your truck fitted with a mast tilt indicator so you know for sure that the forks are level before you pick up or put down a load.

There are several types available, but the best by far is the SUMOLevel available from SUMOSafe.net. Not only is it the most accurate, allowing you to level the forks to a fraction of a mm along their length, it is also very simple and clear to use.