I get asked almost every day "Which height front sight do I need to go with such and such rear sight?" The sad and sorry truth is that no one knows for sure and anyone that tells you a specific height is just guessing.
While it's possible that you will need a different height front sight to go with your new Harrison Design rear sight, I can't tell you what that height should be. All 1911s are different in how they lock up and with all of the different makers of both guns and parts and all the different barrel lengths, ammo combinations and shooter's techniques, there are just too many variables to give you an accurate and honest answer to this question. Since your windage corrections are all made by moving the rear sight from side to side in it's dovetail, what follows below is only pertinent to getting your elevation zeroed.
I recommend that you install your new H-D rear sight, then go test fire the gun with the existing front sight in place, if possible. Shoot a test group at an accurately measured distance like maybe 15 yards, using the sight picture and hold in the adjacent picture. I would suggest for maximum accuracy that you shoot from a bag on a rest and shoot an accurately fired group using 3-4 magazines worth of ammo, all on one target to get a really representative group of where you, your gun and your ammo are hitting.
A round dot on your target about 3" in diameter will appear about the same width as your front sight at 15 yards. Once you find the center of your group and measure the distance from it to 6:00 on the round dot, you'll know what the elevation (up & down) error is that you need to correct. Note that we are using the 6:00 point on the dot to aim at because it is a more refined sight picture than trying to hold your sights where they are half-way from top to bottom of the dot. We are not setting up your gun for a 6:00 hold. This is point of aim = point of impact. I myself, would prefer to have my group impact just at the top of the front sight, so I do not obscure my point of aim with my sights.
You can calculate your theoretical front sight height by multiplying the sight radius, times the error on target, divided by the distance to the target, all in inches. The sight radius is the distance from the rear aiming surface of the front sight to the rear aiming surface of the rear sight. For example, a Government Model pistol with a 6 7/8" (6.875") sight radius that shoots a group 3" low at 15 yards (540") looks like this: 6.875 x 3 = 20.625 / 540 = .038" shorter front sight. Before you go buy a new front sight, I would suggest taking the extra step of shortening your old front sight by about 80% of the correction needed and test fire again, so .038" x 80% = .030" shorter to start. Go back to the range and shoot another group to confirm how much change you still need. This will let you creep up on the perfect height to get your gun to zero.
Of course, if your group on the test target is above the 6:00 position on the dot, you need a taller front sight. Follow the instructions below to be able to creep up on the perfect height, or calculate as above, add the change in height needed, add that to the existing sight's height and go buy a new ready made sight as close to your measurement as you can find.
If your pistol has no front sight, like a build from parts, or your front sight is so short you can't accurately see it like an old Colt, your best bet is probably going to be to install a plain black new front sight that's obviously too tall, like in the neighborhood of .215". Then follow the above instructions and shorten the sight per the instructions to get your elevation zeroed.
