I have this great mulberry syrup I found. I want to prime with it, and I forget how to figure this out with an unknown sugar source. I see that John Palmer's "How to Brew" references measuring the gravity of honey (a variable ingredient) to figure out priming rates in How to Brew, but he mentions you could dilute it and measure it with your hydrometer.
So, say you have some awesome backyard beekeeper's honey or other lovely sticky stuff you want to dilute into a priming liquid. What steps do you take, and what's your math to figure out how to get, say, 2.5 volumes of CO2 into a 3 gallon batch of beer? Or even a 5 gallon batch.