Goats are one of the toughest critters on earth to field judge. When a 9" billy is considered big and a 10" billy is considered huge I think you get the picture? Just like any horned animal, mass really adds up in a goat's score. There are 8 mass measurements on horned critter's B&C score and there are only 2 length measurements....so length isn't everything!
To make B&C you will likely need both length and mass. If you look through the B&C books you can get a feel for what size mass and length measurements it takes to make minimums.
One of the toughest things to figure out w/mtn goats is how to determine nannies vs billies. If you look through the B&C books there are very few nannies that have ever made it into the books because they usually are a lot thinner horned...x 8 mass measurements and that should tell you how important mass is! If my memory is correct there is a 12" nanny in the B&C books so it isn't impossible, but very improbable making B&C w/a nanny.
Trophy quality can vary greatly depending upon where you will be hunting goats. If you drew a Colo tag there are very few Colo mtn goats that have every made it into the books; however, I believe the current P&Y World Record was shot here in Colo. It amazes me that there hasn't been any other goats in Colo that have come even close to that particular goat's score?
Trophy is in the eye of the beholder and to many goat hunters a trophy may be hair quality rather than horn measurements? I really don't like the look of early season mtn goats so to me a trophy goat would be one with long, luxurious hair!
After you have picked out your trophy goat you also need to take particular care to make sure your goat doesn't have a broken tip and also try to figure out where your particular goat will land once he is shot. There are numerous horror stories of goats doing death dives off cliffs after the shot and ending up all scared up...sometimes w/broken horns!
There is no way you'll be able to trophy judge mtn goats without a good spotting scope. If you can get within 100 yards and use your scope this can really help determine mass and length measurements. I usually try to remember that a dollar bill is 6" for judging game. If you are fortunate enough to live close to where you'll be hunting and can judge goats prior to the season at close range through your scope...so much the better! The more time you can spend watching goats and the more you look at the better you will become at judging goats in your particular area. After looking at a bunch of goats once you see a giant billy the big'un should stand out!
I haven't even begun to comment on judging particular horn measurements. Some guys use ear, nose, or eye length in comparison to many of the measurements. One thing you need to remember is that those particular measurements can vary greatly depending upon the maturity or size of the particular goat you are looking at. I have never been a fan of using....say nose length for determining horn length or eye width for base width but some guys swear by this. If there are several billies in a group it is a lot easier to compare horn length and mass than a billy off by himself. More times than not guys can get really fooled sizing up a single billy that is off by himself!
I've just kind of scratched the surface and could go on and on about goats but I hope some of the things I've mentioned will help? My biggest word of advice is to get out there and look at as many goats as you can prior to the season! After doing this...when you see a mondo billy you will definitely know it!
