Friday 15th -- flood risk rising, milder

I predict gusts partly from climate pattern analogues, in other words, different stability profiles or wind directions give different ranges of wind speeds.

Normally gusts will be about 1.6 times the sustained wind (a sustained wind is the highest 2-minute or whatever time used average in the recent time before a report, and the gust is the highest wind speed recorded instantaneously in the hour before). This tells you that wind speed can also fall below the reported wind speed.

Anyway, an example of the general rule would be 30 mph gusting to 48, or 50 gusting to 80.

I might consider narrowing that range if the air is expected to be stable near the surface because then it won't mix down the higher wind speeds as much. In continuous warm advection sometimes you see a report like 40 G 48 which would be 20% and that's unusually small for a gust increase over sustained.

Winds don't gust very much when they fall off below about 15 mph in stable conditions.

As you say, terrain influences gustiness, it can push up the ratio to about double in some cases (as in 30 G 60). If you ever see a report that gives a very high ratio, like 20 G 80, this is because a squall has occurred in the past hour and winds are already decreasing considerably.