First you need to have a magnifying glass or good eyes, ha. The way we used to figure "drag" surface (old school), bearing length, is;
1. determine the rifle bore diameter, not the land diameter ( Most 30 cal. are 300, I have Krieger match barrels that are 299.5, so this is a determining factor)
2. with a height gage put a straight wall cylinder gage on top of your bullet, and the bullet standing vertically square and the cylinder gauge vertically square measure the length of the bullet that is below the cylinder and record measurement.
3. with the bullet sitting on the straight wall cylinder, vertically square, measure this and record measurement. (no bottom measurement is needed for flat based bullets)
4. most will tell you that comparators should be used but unless you have a straight wall cylinder comparator that is cut to .300 or the exact bore diameter they will not work effectively, most comparators have a negative ogive cut into them and will not fit the exact ogive of every bullet and even most straight comparators will have a very small chamfer so they will not measure exact. a straight wall cylinder gauge is cut sharp to the exact bore dimension. Remember that comparators are for comparing bullets using the same comparator not measuring bullet bore diameter. (this is why when you measure your bullet with comparators they will be very different from factory specs. Remember to subtract the height of the cylinder from the height gauge when you subtract these measurements from the bullet length, what you have left is the BSL (bearing surface length),
5. formula: (bullet length - top measurement - bottom measurement) = X, then X times 3.1428 = BSL
Hope I made this clearer than emulsified humus- (mud)
Happy shooting!
