a very quick, crude estimate based on a cam-rocker distance of about 200mm, with a temperature increase of 100 degrees above cold engine temp, the rocker grows about .234mm from cold to hot.
So, if you open the COLD length up to .3 - .4mm you should be alright to start it.
No flaming! - that was a very fast dirty calc, not taking into account the rocker posts/head growth etc.