There is a chart floating around, I believe that Jimbo initially posted it at Aquaticplantcentral.com that coverts the Seachem dosing schedule to a 2 or 2.5 gallon tank. LeftC may also be able to provide a good conversion chart.
Personally, I found that when I tried the Seachem dosing on a 2.5 gallon hex tank, the results were not as good as just dosing a pinch of nitrates, phosphates, potassium, calcium and epsom salts weekly along with a about 1 ml of trace element mix. It was less of a hassle, cheaper and the plants seem to do better. The weekly dosing with dry ferts goes hand in hand with Tom Barr's low tech approach of setting up a tank and in my experience works just as well when tapered down to 3 gallons.