Its a strange one TBF . Most of the Bird Of Prey have a optimum Temperature of 99.5F/37.5C, the Grumbach ticks over nice in the Incubator room just under this around 37.2 - 3 which I feel happy with and the Hatch rate. Temperature/Humidity/Turning all play a big part in the Hatching of eggs! some eggs are also much more temperamental than other's and need to be weighed such as some of the BOP eggs.
If I'm weighing eggs you take the birds fresh egg weight and aim somehwere for a loss of around 15% to pip, but 13% suits me fine, basically a egg needs to loose fluid to hatch which is effected