I just measured the 'factory blade ' I have in my bin at .475ish (no idea what caliber it may have been intended for) from under the base to top of blade. and the one on my 22LR Sportster is shorter but it was filed in previously. A blade suitable for the mid-barrel open sight will be shorter than one suitable for the breech mounted peeps.
I dont recall a list of various factory blade heights for the different calibers in the FAQ's......maybe we should start one?
FWIW, I have used the 'run a level from the front sight' method in the past and got reasonably close. You can whittle out a simple wood sight or bend some flat alum. in an 'L' shape c/w notch then elec. tape on for tests too.