I am about to embark on the construction of a built in bed, and want a
support leg half way along one side.

I was going to joint in a piece of wood, but was a bit doubtful about
getting millimetre precision with my dodgy woodwork.

This is the joiners method for leveling up:

Build the whole bed, with legs all the same length.

Assemble the bed in the location you want it permanently.

Put wedges under each leg as necessary to bring the bed level and all
legs into firm contact with the floor.

Find an offcut of wood that is slightly thicker than the leg raised
highest from the floor.

Slide this offcut around each leg, and use a pencil on the top face to
mark how much has to come off each leg.

Now when you saw the legs to length, it takes account of the floor
irregularities at the intended location.