It tends to work out because my efficiency is fairly consistent. I essentially plan my grain bill for 70% collection, knowing the I could probably achieve 75% by collecting more wort. Similarly, I plan to hit that efficiency by collecting 0.6 to 0.65 gallons per lb of grain. If necessary I can collect a bit more, but then I might start to watch the gravity of the runnings. Finally, I plan for a 5.5 gallon batch so that if my efficiency is higher or lower I can readjust my final batch volume accordingly and still hit my planned OG. So, there's not a ton of precision, but a lot of room for on-the-fly adjustments.