Calculating the correct amount of speise

Papazian's The Complete Joy of Home Brewing provides a formula for speise (gyle) that assumes a priming rate of 3/4 cup of corn sugar per 5 gallons. Snyder's The Brewmaster's Bible provides a table which assumes a five gallon batch. Neither source takes into account the residual carbonation from fermentation. BeerSmith2 does, but you can only choose from corn sugar, DME, or honey. It doesn't have a speise option. Can I possibly use the amount that BeerSmith2 gives me for DME to calculate the amount of speise that I need? I figured that I could do it this way:

If BeerSmith2 tells me that I need 0.416 pounds of DME for priming and it thinks that DME has a yield of 44ppg, then I need 18.3 gravity points to prime my beer (0.416lb*44pts/lb). I have 5.19 gallons of beer with an original gravity of 1.076, which corresponds to 76 pts/gal. 18.3 points divided by 76 pts/gal. equals 0.24 gallons of speise. In other words, a quarter gallon of unfermended wort with a specific gravity of 1.076 will prime my beer the same way as 0.416 pounds of DME. Right? Have I gone wrong on assumptions or calculations anywhere?

I don't know but this might help
http://braukaiser.com/wiki/index.php?title=Kraeusening

dzlater said:

I don't know but this might help
http://braukaiser.com/wiki/index.php?title=Kraeusening

Click to expand...

Thanks, but unfortunately, that source goes into a little too much detail with regard to CO2 volumes, and thereby seems somewhat prone to error. I just need to convert from a mass of DME to a volume of wort of a certain specific gravity.

I just picked up Papazian's The Home Brewer's Companion. In it, Papazian states that I can convert a mass of grain to a mass of DME at a ratio of 0.9. So...

13.25 lbs of grain (total) * 0.9 = 11.93 lbs of DME
11.93 lbs of DME / 5.19 gallons of wort (into fermenter) = 2.30 lbs of DME / gallon of wort
0.416 lbs of DME (for priming, as calculated by BeerSmith2) / 2.3 lbs of DME / gallon wort = 0.18 gallons of speise for priming

...And that isn't too far off from what I had calculated in the OP. I think we can chalk up the difference to Papazians propensity for approximation. I intend to brew tomorrow. I feel confident enough to try my speise calculation for this batch so we'll see how it goes.