From my early editor days, I’ve expressed thousandths of an inch in numerical form, not as X mils. So, for example, instead of “1 mil,” I would write “0.001.”
After 20 years, I am rethinking that convention. (I keep hoping it will all be overcome by a shift to metric, but that’s not just happening very fast now, is it?) And I am willing to accept reader input. Which do you prefer: mils or numbers?