The ROT is 0.09 gal per lb of grain. That will vary somewhat depending on kernel size, crush, etc. But it should be a decent starting point. Once you've drained your first runnings, it will over-estimate because of the soluble extract you've removed, but that just gives you a safety margin. For your example, an 8.5 lb grist, after being drained, would have a volume of 8.5*(0.09 + 0.13) = 1.87 gal, so once you add the 3.75 gal of sparge water the total volume will be ~5.62 gal.