Ideally the trouser leg should 'break' once or twice before it touches your shoe - just once if you'd like some sock to show. Most men know what leg length they prefer - and these are much more consistent than waists, across all brands.
How to Measure
If you are unsure of your leg, the easiest way to measure is on a pair that fit you well, laid flat. Just run your tape down the inside seam from the crotch seam to the hem. In this case, the measurement in inches is your leg length - no calculator required - and you'll find it probably matches the stated size