I might be way off base here, breaking some sort of rule or something, but I have always used two methods here.
The first is the "Load it in the Magazine" test. If it will fit into the magazine, then it just might be alright. I then go to the second test. This is to visually 'gauge' the loaded round along side of a factory load. If it is of equal height, then it's probabaly OK, shorter is OK too, as long as it isn't obviously compressed. If the reload is 'taller' then the factory round, then I would seat the bullet to at least match the height of the factory round.
Like I said, I'm probably breaking a whole box of reloading laws and regulations, but when I started out, I didn't have access to a set of calipers, and nothing that I read at the time said that it was a dire 'requirement'. The bottom line was that I surmised that if it was at least equal to the factory round, I couldn't go wrong. It was only after a few years of experience and doing research in various reloading manuals that I saw that there was actual spec's for OAL with certain bullets. Ignorance is bliss, I guess.
But I can't help but wonder if my 'common sense' method of coming up with a OAL has any merit? Anyone out there have any similar experince here? Let me know!