The math
Two things are being calculated. The dates are plain arithmetic on a 52-day period. The weight targets are a straight line, because an egg in a stable incubator loses moisture at a near-constant rate, so the target for any given day is just the fraction of the way through you are.
hatch_date = set_date + 52 days
target_weight(day) = fresh_weight × (1 − loss_target × day / 52)
Set a 600 g egg with a 14% target and the line runs from 600 g on day zero to 516 g at hatch, losing about 1.6 g a day. If your egg is dropping faster than the line, the air is too dry and the humidity wants nudging up. Slower, and it wants coming down. That correction is the whole reason to weigh.
Frequently asked
How long do emu eggs take to hatch?
About 52 days, with the usual range running 48 to 56. Emu eggs are unusual in that incubation temperature moves the date noticeably. At 97.0°F (36.1°C) most eggs come off around day 50 to 52. Run cooler, nearer 96.5°F, and you can be waiting until day 54. Run warmer and they arrive early, which is not the win it sounds like - fast hatches tend to bring weak chicks and unabsorbed yolk.
Why can't I candle emu eggs?
The shell is dark green and thick, so an ordinary candler doesn't put enough light through it to show veining. A very high-intensity or infrared candler can sometimes read the air cell late in incubation, but it isn't reliable enough to run a hatch on. Weighing is the standard method instead. An egg losing the right amount of weight is losing it through a membrane that's doing its job, which tells you more than a blurry candle would.
Why is the humidity so much lower than for chicken eggs?
Because the egg has to shed 13 to 15% of its laying weight before the chick pips, and it has 52 days to do it through a thick shell. That needs dry air, usually 24 to 35% relative humidity. The other difference that catches poultry keepers out is that emu eggs don't get a humidity spike at lockdown. Chicken eggs go to 65% for hatch; emu eggs stay low right through pipping.
When do emus lay, and how many eggs will a hen give me?
Emus lay in the cool months, so in most of the United States the season runs from around October or November into March. A hen lays roughly every three days through the season and a good one will give you 20 to 50 eggs. In the wild the male does all the incubating, sitting about 56 days without eating or drinking and losing a third of his body weight.