Easier to just refer for further evaluation, etc., but it seems that, like me, you want to know WHY!!!
Poor duct design? Is that 107 degrees consistent throughout the system?
And one thing that hasn’t been mentioned is that the hi-low limit may be malfunctioning…the kick-in temp for the fan may be too high because of this. They CAN be calibrated, but usually just get replaced by HVAC techs to avoid the possibility of call-backs.
The key to your problem here is that the return air temperature is already too high.
What is your setpoint temperature, 68? The return is 64!
It’s like trying to figure out temperature drop in an air conditioner in the wintertime when the heat is off in the house!
Forget about all these diagnostic formulas. Just turn it on and see if it runs.