As it’s a triple, then it will most likely have a 120 degree crank, so think it best to follow the manual. If you cannot get to see timing marks, then the only sure way is to use a rod inserted to rest on the piston top as discussed earlier.
However, my other concern is how you intend to “inch” the engine. My experience of long ago in attempting to do that using the starter is extremely hit and miss…more often than not: miss! A spanner on the crank is best or alternatively rocking it in gear, but that may not be practicable with the weight of the digger.
So failing all that, my thoughts return to your original thoughts. If you set them when they show maximum slack then it should be fine, but as said, the big issue is being able to inch it gradually. If you cant get to the crank, might it be possible to get a spanner on the alternator or fan pulley, as long as glow plugs are removed to release compression of course.
Alternatively, there’s the old adage: if it ain’t broke, don’t fix it!
Good luck.
