Where did you get that number ? I got the original repair manual with my R 69 S from 1965 . There are all press fits for any engine and gearbox components . So for a 38 mm inlet valve the seat has a shrink fit of 0,20mm , only the smaller 34mm exhaust valve got 0,15 mm shrink fit = 5 thou. That has worked for decades, never dropped a ring. These numbers correlate to the sizes you get when heating the head to min. 200 degrees. So you will be just able to drop the rings in . So same calculated shrink fits were applied to my Vincent heads, wouldn´t want to do a lighter fit. Anyway you have to maintain a certain percentage of shrink fit depending on basic valve sizes, smaller valves less , bigger valves more shrink, so no fixed numbers.
Certain cast iron seat rings were quite allright while leaded fuel was available. But today you have to offer rings that have to operate with diesel, liquid gas, ethanol etc. so no easy task now.
Vic