How much tetra aquasafe plus do I use after a water change?

I have a 10 gallon fish tank and I take out about 2.5 gallons of water and fill it back up with 2.5 gallons in a water change. I add conditioner with the 2.5 gallons before I put it inside my fish tank. The directions say to apply half the cap/1 teaspoon for every 10 gallons and 1 whole cap for every 20 gallons. If I do a 2.5 gallon water change, do I use half a cap of conditioner because my tank is 10 gallons or do I use 1/4 of that for the 2.5 gallons of water?