Take the sea-level pressure. Subtract 1000. Multiply what's left by by
0.8. This is the height of the 1000-mb level in decametres. (It could be
negative). Subtract it from the 500 mb height. There's your thickness in
decametres.
The factor of 0.8 is not exact and depends on the pressure and
temperature. 0.8 is exact at 4°C and 1013 mb SLP with dry air. Strictly
speaking it is proportional to the temperature (in K) and inversely to the mean
pressure of the air between sea level and the 1000 mb level i.e. half their
sum). Ignore these niceties; 0.8 will do. Some of the figures quoted in
another thread are too high.

Tudor Hughes, Warlingham, Surrey.