Does this old trick work?
 

Wondering how high the cloud base is?

Temperature - dew point multiplied by 400.

E.g. temp 15°C - dew point 11°C = 4 (i.e the difference)

4 x 400 = 1600

The cloud base is likely to be at 1600ft.

Try it out!

I never think about it. The ATIS temp and dew point at departure should give some result, but flying in Scotland I see big variations in cloud base on cross country. After crossing hills, and dropping water, there must be big variations in humidity in air "packets".

Yes, the trick works, but for cumulus clouds only.

Inbalance

Actually, it is a very rough rule of thumbs and it is not exactly 400 ... ;-). Yes, it does work pretty well for mid-day and not too extreme water content in the air and temperatures away from dew point (and btw, is taught in basic weather class for PPL ;-) ...). Do not use it in early morning conditions.

In my experience it's totally useless. It's wrong more often than it's right.

Fine if you have your own wet and dry thermometer to take the measurements yourself. Somewhat pointless if you are using an external info source (such as ATIS or Met Office weather) for temp and dew point as they will also provide cloud base info (unless CAVOK of course!)

The problem is in the assumption that temperature falls linearly with altitude.
This is normally true but fails when conditions are changing such as morning and evening. If the thermal gradient is perfect the glider pilots get miserable.......