if 100.4 is considered a fever with the “norm” generally regarded as 98.6, then a 1.8 degree temperature rise is what’s considered a fever.
For my daughter, that number would be (comjng from 99.5, which is her average resting temp) - 101.3. Far above the 100.4 limit. 100.4 for her is like someone with a resting temp of 98.6 having a 99.5 degree temperature - which, as you stated, is not considered a fever.
for myself, on the other hand - with a 97.5 temperature, my “fever” range - going with the 1.8 degree rise - would be 99.3 - far lower than the 100.4 threshold