The beauty of the pragmatic method is that you don't care so much what the absorption is. After your first runoff, you simply measure how much more you need to get your pre-boil volume, add that to the mash, and runoff the second time. Done.
If you want to do calculations, you could measure your first runoff and simply subtract that amount from whatever total amount of water you had added to that point. The only problem is that that doesn't directly tell you your absorption amount. The calculated result will be the sum of the absorption and the dead-space in the tun. The dead-space won't change from batch to batch but the absorption will. In general, you're best off just using something between 0.10 and 0.12 gallons per pound of grain to calculate the absorption.