what is the best way to measure for shaft length
The correct way to fit for the correct length, is to use face decals, and hit different length clubs determining which length creates the most consistent pattern, on irons and woods.
https://www.golfworks.com/oversized-iron-impact-decals/p/gw1041v/
https://www.golfworks.com/oversized-driver-impact-decals/p/gw1042v/
As far as marking shafts to cut to a specific length, there are a couple different ways.
https://www.golfworks.com/maltby-bench-top-club-length-ruler/p/ma2010/
https://www.golfworks.com/the-golfworks-48-aluminum-golf-club-ruler/p/rul48/