That 15 degree rule is real and most folks who camp a lot know it. Temp ratings on sleeping bags come from the EN 13537 test, and the number they slap on the tag is usually the "limit" rating, which assumes you're a warm sleeper in a base layer on a pad with an R value around 5. The "comfort" rating is what matters for most people and it's typically 10 to 15 degrees higher than the limit. So a bag rated 15 is really a comfy bag down to about 30 for an average guy, and if you sleep cold it's worse. Synthetic fill that clumps is a whole other problem, once it balls up in the corners you've lost loft and loft is what keeps you warm, no rating saves you there. My rule is buy 20 degrees below the lowest temp I expect to see, and pair it with a pad rated at least R-4, because the ground steals more heat than the air does. A $40 fleece liner adding warmth tells you the bag itself is the weak link, not your setup.

Crank desks usually fail because of cheap gears, not because you got lucky with controls.