Proper yeast volume for 2.5 gallon batches?

I've done 4 batches at the 2.5 gallon size, mainly because I was gifted with a Mr Beer set but never liked their stuff, so I got a 3 gallon better bottle. I had trouble with my first two batches tasting extremely yeasty, even months later. I realized I had halved all the ingredients in my recipe except my dry yeast packet, so my third and 4th batches got a half packet, and they turned out well.

Advice from the LHBS was to just use a full packet, and that "it should have worked" but my limited experience says otherwise.... what do others do? I found links to Mr. Malty's yeast pitching calculator that says I am right at using ~a half packet.

For my latest batch I bought a vial of liquid yeast, and mr malty says to not use a starter. Does this make sense?

Depending on your recipe it would.

Yes.. Follow whatever the mr malty calculator says. Dry packets contain way more yeast than liquid vials. You should probably pitch the entire vial, but won't need a starter.

You may also want to let the beer sit up to a month after fermentation is complete so more of the yeast can fall out of suspension. If you had another carboy you'd just rack it to secondary after fermentation was complete and wait a month.