Syco, what I used to do with my 600 was use a 17mm socket and a ratchet on the end of the crank to get the fine adjustment required to line up the T mark.
What you are doing with the feeler guages sounds ok as long as there is a little bit of resistance when you move the feeler gauge,you should be right.
It is the Auto decomp that causes the issue with it binding on the RH exhaust valve not the manual one.
When the decomp used to bind up on the right hand exhaust valve, I would use the kicker to turn over the motor a full revolution, line the T mark up again and it should be good. Another trick I have used is to use the ratchet on the crank and turn the motor back wards (anti clockwise) just a tad,you should hear an audible click as the decomp dissengages.Once it clicks, line up the T again by turning the motor back the other way (clockwise). I have heard that it is not a good idea to turn your motor backwards but it does work and the motor never seemed to suffer from doing that.
Are you talking 8mm free play on the lever that the decomp cable connects to on the rocker cover?
