Used to do it on Whirlwinds (three Main blades) up to the early 1990's - using a tripod on top of which was a turnable pole. Total height up to about 12 foot tall.
On the side of the pole were two perpendicular legs about 2.5 to 3-foot apart between which was a 4-6 inch strip of canvas mounted on 4 springs.
We would colour the blade tips Red/Yellow/Blue with Wax Crayon. We would also stick masking tape to the edge of the Canvas (so that we could see our marks and not all the others!)
We would set up the positions of the tripod feet on the floor and the approximate angle of the pole to the blade prior to running up the heli (holding up the blade to meet the flag) that took a lot of guesswork out of it!
Jockey would run the heli up and give us the Thums-up when he was ready. We would move the Tripod/Pole assy into place and he would hold the Disk at the mid-canvas level until we'd either told him to stop - or the tape disappeared with someone chasing after it!
After a couple fo ruins we could achieve splits as little as 1/8th inch. Then send it for Flight Checks (using a Scientifica Atlanta clockwork mechanism with Red "wax" tape - but thats another story!)
Happy Memories!
Rigga