This has worked well for me:
- Treat and heat a little more than the batch size of sparge water; e.g., maybe 6 gallons for a 5-gallon batch; this will probably be too much, but better to have too much than too little
- Measure first runnings
- Do a first sparge of half the difference between the first runnings and the total pre-boil volume, plus half a gallon or so "just in case"
- Measure first runnings + first batch sparge
- Do a second batch sparge of remaining volume needed to hit total pre-boil volume, plus half a gallon or so "just in case"
- Only collect enough of the runnings from the second sparge to hit your pre-boil volume on the head
If everything goes perfectly, I end up wasting a little bit of the second batch sparge, but at that point, you'll be down to 1.020 or lower, so, not missing out on that many fermentables, and I like having a bit of margin of error.