the hali should stay docile once on the surface as long as you don't pull it's head out of the water. I was on a trip to the charlottes in the early 90's with the halibut commission and we were tagging and releasing them off of hippa island, it was non stop pretty much and some were up to and maybe over 200#s. talk about a work out, lol, sure coulda used something like an ace line hauler, it was a brutal few days, sure couldn't do it now.
as for the measuring, why not just tie a swivel on your line at 126 cm from where the hook is on your setup,or from the swivel on your spreader bar or whatever you are using, you could put a red bead there for better vision, once the hali is on the surface and docile just lay the line back towards the tail, no fuss, no sticks, also would make it easier to eyeball if you were having trouble bringing it alongside.
cheers nicnat