I’ve long known that the temperature and dew point spread at various altitudes can used to forecast expected cloud cover. What I didn’t know until recently was the rules of thumb used to make the forecast. Thanks to the USAF weather squadron in Germany, I was able to track down that info:

Temp/Dew Point Spread *C at altitude — Forecast Cloud Coverage — Description
0-2*C — 8/8 — Overcast (OVC)
2-3*C — 5/8-7/8 — Broken (BKN)
3-4*C — 3/8-4/8 — Scattered (SCT)
5+*C — 1/8-2/8 — Few (FEW)