I hadn't heard that number, but it seems reasonable. The 2009 "UHR" really can't compare to the original, because the original was truly much higher relief. The rims are always the highest point on a coin, of course, and the different basining techniques can create the illusion of a high relief. I'd wager the 1907 HR Saints were more than 1.4 mm relief.
Everything I've read says the $20 UHR Saint Gaudens were 2.41mm thick. Divide by 2 and you have 1.205mm MAX relief. But you must subtract some minimal amount to account for the thickness of the fields. So. Math.
Confirmed, PF...it was from the little book that came with the coin. Page 43: "The draft angle was established at 45 degrees to minimize collar release problems during coining. Letter height was estimated to be 1.44 mm, and although the lettering on the original coin was not vertically centered, the 2009 UHR DE version was centered after the development of the font geometry."
Clarifying that I was referring to the 1907 high relief $20 double eagles (I mistakenly included U=ultra, which would have been referring to the extremely rare/unique 1907 UHRs).