We counted ours from day 1 and left them for 2 days afterwards. We had candled them at days 6, 12 and 18 (just before they went into the hatcher). We left them 2 days afterwards then I candled them again to see if anything was moving and cracked them open to see what went wrong. Big mistake. I killed one which got cut by a piece of shell and bled to death in my hands. It was moving as well. After that we left all the batches another day but didn't get any later hatches. Turned out then the Suro incubator was running lower temperature at the sides, even though the thermometer said it was OK, which it was in the middle. The small print with the Suro said it had to be in a draught free area above 20 degrees, so next time we will run the unit inside a purpose built chamber as 20 draught free degrees is impossible in an 1830's house in March.
So Craig, I would make sure no infertile or dead ones go into hatcher (because if they break the bacteria in the rotting egg will kill the hatch) and leave them 3 days extra, or 2 days extra as Foxy says counting from day 0. If any start to weep (small crusty deposits on the surface) remove them very carefully immediately.