Can't find Seans forum name atm, but I wasn't aware he had an online version. Would have made my work a lot faster.
Don't worry, I found you!
I'm using a similar approach now, but when calculating lauter efficiency by the ratio of volumes, you need to account for the volume displaced by the sugar during the mash, which he appears to be approximating using the C=1/SG coefficient. I'll need to check out his source code, as my calculator, which matches kai's to a couple decimal places (floating points yo) is calculating a difference of 3-6% lauter efficiency. When I lower the absorption rate to 0.12, they're pretty close though (.5-2%). Which matches the apparent absorption rate using preboilvolume - strike / grist mass.
That all sounds correct to me, but to be honest I haven't looked at my own source code in 3+ years. I'll try to dig into it a little more tomorrow.