I find it's easiest to get a commercial (or previously brewed) similar beer, and mix in extracts to see how well they match up and what quantity to use. 32oz is a lot to sample with, but you could get a few commercial blondes and mix, then taste it. After that you'll know how much to add. I've done this with a few to see how well they work out.
Now, I know that a lot of places sell extracts designed for 5 gallon batches. If that's the case, you could very well split the bottle in half and add accordingly. To make it easier, you can bottle 1/2 of your 5 gallon batch, pour half of the extract bottle into your bottling bucket, stir, and bottle the rest. The only problem is that not everybody prefers the same strength of flavor.