I've heard the figure of 5 per gallon, but that's not necessarily correct. In your case that would be 155 x 5 =775 !
My suggestion is to add them in "groups" of aprox 25 per group over a period of time. You should visually assess how well the snails manage the algae, how much ground they cover etc.
Most likely it will work out to 2 - 3 per gallon (if you use the per gallon method) Also keep in mind that snails vary regarding their hardiness, some are very hardy, some die quickly. You should acclimate them as you would a delicate fish (described in my book)