In my best cooking/baking experience (and looking on the back of the lemon extract box :) ) you can substitute 1 t. zest for 1/2 t. spoon extract - or vice versa.

You can't - God has yet to make a "standard lemon".

The best idea is to work from the suggestions for use given with the extract - or hope that someone with experience of using both comes along.

My apologies, that's not me ...

Just looking at the box of fruit extracts I just bought... It says that you need 1/2 teaspoon per cup of dry ingredients, and 1/4 teaspoon for cup of liquid. Most of the suggested recipes on the box seem to need 3 teaspoons of extract. I would expect that to be roughly the same as 'the zest and juice of 1 lemon/orange' instead.

Amazingly a 1 oz bottle apparently contains 44 oranges, 66 lemons and 80 limes (depending on the flavour!) so I guess a tiny amount has a pretty profound effect!