Are you just using a single smack pack per batch, or are you doing starters? If not doing a starter then you may actually be better off with dry yeast. The calculator at mrmalty.com tells how much to pitch for both dry and liquid yeast.
Many folks just sprinkle the dry yeast on top of the wort but there will be some cell loss that way. I usually do that with my 2.5 gal batches because an entire pack would be more yeast than I need anyway. To avoid the cell loss you can rehydrate the dry yeast before pitching, see below.