Follow the Brinsea instructions. They'll tally with the incubator. I've got two Brinsea Octagons which I imagine would be the same and it'll certainly be 37. something rather than 39. something. If you want to be really sophisticated, you need to measure weight loss - you weigh them at the start and then every few days to check they're losing the right proportion. TBH, I don't normally bother with chicken eggs but for harder to hatch waterfowl it's worth it.