OK, chaps, this is what I envisaged. As you can see the level of the driver's eyes bears no relation to the digging level required, so any suggestion based on a type of sight gauge is a non-starter.
I was envisaging some sort of simple receiver on the dipper, just a lens or reflector of some sort and I was hoping for some simple beaming arrangement as shown in the sketch top right, the middle zone needn't be too accurate - plus or minus an inch or two would be good enough. Doing it by eye I can't keep level very accurately, can easily be out by 3 or 4 inches in either direction after a couple of yards, so a couple of inches over the whole excavation (10 yards or so) would be pretty good.
I'm quite happy to use a PAPI as I think I understand from PEI's description, but I don't understand (well, can't remember from school 50 years ago) what sort of lens would throw the required beam out with a 90 deg (or thereabouts) spread in the horizontal plane, and for some reason giggle seems to throw up pages and pages of stuff about camera lenses, nothing as basic as this, so any advice on this would be appreciated.
Cheers
UFO