I've always used the same rule of thumb for stock length of pull as described by Dean all I would say on that is that the measurement from the crook of the elbow along the forearm to the trigger finger is to the pad of the finger as if it were on the trigger blade i.e. the finger is curved.
An issue I used to find though with adding spacers to the butt of a 'standard' stock is that the cheek piece then ends up too far forward on some stocks for a comfortable cheek to stock fit, however this may only apply to me as the LOP I need to have on my stocks is 17 1/2 inches some 3 1/2 more than standard.
Thus I had my last stock custom made to measure by 'GinB' and they did it with the longer LOP and more importantly with a correctly positioned cheek piece. Apparently it's the longest stock that they have ever made !. They also made the pistol grip to my hand measurement so the whole tthing fits like a glove. All done via e-mail as well considering they are based in Bali.
The downside to this of course is that the stock basically fits me and others who have tried it do struggle with the LOP.
As with any made to measure stock selling it on afterwards may be a problem.