I know this is a bit late, but I thought about running sizes and batch sparging a bit and came up with an analogy that makes sense to me. Imagine that you have a kettle that always leaves 1 gallon behind when drained. So if you have 200 'points' of sugar and dilute it to make 5 gallons of wort, that's a 1.040 wort. If the kettle is drained as much as possible, 1 gallon of 1.040 wort will be left behind, or 40 points out of a possible 200 (20%). This is analagous to a no-sparge.

Next collect the same amount of wort in two steps, first by collecting 3 gallons then collecting 1. For the first runnings the wort will be 1.050 gravity, leaving 1 gallon behind. Adding 1 more gallon of water will dilute the wort to 1.025, again leaving 1 gallon behind when drained. So here you leave behind 25 points out of a possible 200, or 12.5%. This is a single sparge with unequal runnings.

Finally, collect the same amount of wort in two *equal* steps. This is done by a 3 gallon wort which is drained, followed by adding 2 gallons and again drained. The first draining leaves 67 points behind, which is diluted to 1.022 by the second addition, and again leaving 1 gallon behind. Here you leave behind 22 points, or 11%. This is a single sparge with equal runnings.

You can see the difference between equal and unequal runnings (1.5%) is relatively small even for a large difference in runoff sizes. The size of the 'dead space' at the bottom of the kettle can be represented in your MT by the amount of wort absorbed by the grain and any dead space, easily measured on your next batch.