Does anyone have the REAL theory on the blower off delay? What is the most efficient gas, $$ and equipment saving methodology to set the blower off delay?

Old 2000 sqft house, 2 story, 4" walls, half assed insulated, 90% bryant 350mav propane furnace cycles every 15- 20 minutes. Outside temp 30 deg. Furnace mounted inside house, pulls air in withing 3' of blower. Flexible slinky 8" insulated ducts in crawlspace with footing vents blocked with styrofoam.
Furnace has selectable blower off delay of 90, 135, 180 and 225 seconds, comes from factory with 135 default.

Thermostat set at 70 deg.
Heat rise is 108 deg. At vents.

When tstat signals off, heat drops
At 60 sec. 90 deg. At vent, feels luke warm
At 90 sec. 83 deg. At vent, feels cool
At 135 sec. 78 deg. At vent, feels cold
At 180 sec. 75 deg. At vent, feels cold

The blower off-delay is adjustable and applies to heating AND cooling. Typically, the bigger the unit/system, the longer the off-delay. This is to take advantage of the "residual" heating and/or cooling effect. In most applications, the factory default of 90 seconds is optimal. Very "pursuant" on your part to actually measure/time the heat output during the off-delay period. You no longer qualify as a "typical" homeowner. ;)