Thanks for pointing that out. I must've screwed up typing up the recipe. I just double checked the recipe in BeerSmith and it says 8.41gal. All other ingredient amounts are correct. I have corrected it above.
I didn't have a problem with the cough syrup flavor, but we did a little taste experiment prior to coming up with the amounts of extract before bottling. It basically went like this:
Before bottling, took 4 cups (or so, don't remember how many now, it was awhile ago) of the beer out of the bottling bucket, split that in 16 samples.
To half of them (group A), we added the chocolate syrup. Using the "eye'ometer", added what I thought it was a good dose of syrup to sample A1, 1/2 of that amount to A2, 1/4 to A3, 1/8 to A4 and so on.
Did the same for the cherry syrup on group B.
Tasted each group and picked the best from each. So say I ended up picking A4 and B3.
Mixed the contents of A4 and B3 and tasted that to make sure it actually tasted good. If not, then decided if more chocolate/cherry was needed and mix the samples of each group to increase/decrease the mixed flavor as needed.
Once the choice was made, a volume of extract was calculated by the ratio of volume of extract to volume of beer of the test samples and just scaled that up to the remaining volume of beer left to be bottled.
So in the end, I was able to detect the medicine taste before ruining the entire batch.
Initially I thought that the taste of cherry/chocolate was going to be affected at least a little bit by the effects of carbonation on the tongue, but if it did, it ended up not being significant enough to be noticed.