Timing chain - 1 shim too loose 2 shims too tight!

The topic title says it all really.  I felt the timing chain to be too loose so removed the magneto and added an additional shim - there was one there already.  Having added the second shim to chain is definitely too tight - can barely join the chain again.

Leave it at one?  I'm not sure how loose/tight it should be.

It's a Lister D and the RS1 magneto.

Thank you.

I like to be able to have around an inch of total movement in the centre of the chain run, also make sure the sprockets are dead in line with each other as even a slight variation will affect the tension. However if the chain is worn then getting the 'right' tension will be almost impossible.

Pete.