I can't be alone with this situation. Every dry hopped beer I have brewed has been a bit of a guessing game on the final volume in the bottling bucket. I estimate my volume, calculate my priming sugar, transfer, and end up with less than I calculated. It just happened tonight where I thought I would get 4.5 gallons but ended up with 4.1 gallons. I used 4.4 oz of sugar and now I have to worry about overcarbed beer. I was shooting for 2.5 vol. and now it calculates out to 2.7. Seems high for an IPA but I guess I'll have to wait and see. Any tips on avoiding this in the future. I either have to pick the low end (2.3 vol) for ales or maybe just get enough batches under my belt to better estimate the final bottling volume.
You could see how much beer ends up in the bucket and then add a proportional amount of your priming sugar solution. And stir gently.
That seems too logical and simple. I may have to. I've always put the sugar water in first then siphoned into it.
I had the same problem early on. What I do now is make up a priming solution and add it to a measuring cup. I make enough for 5g. Then I pour in the right amount.
Another easy way is to shoot just under your desired volumes of CO2 - I usually add in enough sugar to be on the lower/moderate end of the scale appropriate to style, just to ensure I won't have bottle bombs.
When it's all said and done you're trying to get close. Your 5.26 gallons of beer is going to carb up to ~2 CO2 volumes the same way your 5.00 gallons will be about ~still 2 CO2 volumes.
pi (3.14 is good enough)* radius ^ 2 (or diameter ^2 / 4) * height is the volume in your fermenter. Use google to convert from cubic inches to gallons. If you have a better bottle, with a large indent in the base, consider removing a half inch from the height.
I think the general consensus is don't estimate, measure and calculate based on the amount of beer you net in the bottling bucket. And measure that sugar in whatever form you use by weight. You will be much happier with the results.