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.