Well, fed up with my extending marks wearing off (tape, paint, even etch marks) I have finally found a way of marking my pole sections so that they don't extend too far and fly out of the clamp on a three story property.
I have drilled through the pole with a small drill bit on the top of the pole shaft. Easy to spot, doesn't wear off - as soon as I see it I push the shaft back into the clamp another centimetre. Seems to work ok so far.