learn whatever algae(s) you have in your tank along the glass, rock and substrate and buy accordingly. you don't need what they recommend as far as numbers. for example, i have 15 total snails in my 120 gallon, 2 emerald crab and 3 conches. i have no need to add any more or i'll experience losses due to starvation down the line.
whoever it is that suggests 1 snail/crab per gallon has no soul.