Prop pitch done! (First attempt anyway!)
I made a tool to hold a laser pointer which I bought on Ebay for $17. I think it might be slightly illegal as the damn thing is very powerful - you can clearly see the beam in low light conditions and even in broad daylight you can see the spot on a tree that's 100m or more away.
Anyway, I was interested in @IBob's method of attaching a piece of wood and aluminium to the prop, but being someone who likes to over-complicate things I 3D printed a holder for the pointer based on measurements of the prop blade at 300mm from the tip (600 from the centre). It slips on and stops at the same spot on each blade, and features a sliding switch which holds down the spring loaded button.
If anyone has a Bolly 3 bladed prop of the same model and wants to print their own, I'm quite happy to email you the STL file.
I ensured the prop is as vertical as possible, found the height where the blade is parallel to the ground and leveled the flat back of the blade vertically. From this point I turned on the laser and made a cross on the floor where it hit.
Then used a calculator to get the adjacent length of the right angle triangle where I knew the vertical length and a 19 degree angle. Marked a line at that point forward of the mark on the floor. (Just to check, I calculated for 18.9 degrees and 19.1 degrees. Each 1/10th of a degree is around 2.2mm.)
After that it was just a matter of swiveling the blade until the laser line exactly hit the front line, and repeat for the other 2 blades. I nipped up the bolts just enough to hold the blades from turning - less than 8nm. When I recheck with the spinner thingy off completely I'll tighten up the bolts to the recommended torque settings in the recommended order.