H11 is 8,5, so H12 should be 9mm.
I'm not sure I follow the logic. Are you saying that each "H" number represents .5 mm? Mine has a 3 on it and looks to be 3.5 mm in height, fwiw. To continue on that line of reasoning, H11 would be 8 x.5 more than a number 3, or to turn it another way, 11 x .5 would be 5.5, but neither are the case.
Seems like to get an accurate answer to the question how tall is a sight with the number 12 on it, you need to measure a number 12 sight. You don't need a micrometer or caliper for 1/2 mm, a piece of card stock and a sharp pencil will let you mark the height and measure it with a ruler. But it would be a good excuse for buying a caliper, you will find more uses for it in the future.
