Question on priming sugar vs. beer volume

In Palmer's How To Brew, there are various calculations and nomographs to determine the amount of priming sugar that should be used for a 5 gallon batch. Is that 5 gallon volume referring to the original volume or final volume of beer available for bottling?

I found that after fermenting, transferring to secondary, etc. I was a little over 4 gallons on my first brew due to sampling, sediment, and so on... so I scaled back my priming sugar for bottling to 4 gallons rather than 5 and I found that after two weeks the beer is still pretty weakly carbed. Was I supposed to base my priming sugar off of 5 gallons even though I had only 4? I believe I used around 2.8 oz of corn sugar to hit 2.5 CO2 volumes and scaled that back 4/5th's.

The brew was an American Pale Ale from Brewer's Best.
OG: 1.050
FG: 1.012 (stable over two weeks)

Definitely adjust for volume of beer. Here's a link to use in the future:

http://www.tastybrew.com/calculators/priming.html

Give it more time, and make sure that the bottles are stored in 72-75 F temps.

Just caught that you reduced the 2.8 ozs by 20%, so you only use ~2.25 ozs. You're not going to hit 2.5 volumes, more like 1.75 volumes.