If you are shooting low, then the front sight is too high. Dress off the top of the sight with a flat file until you get the correct point of aim. You can then leave it as is, or measure the height and order one of those. If your point of aim is off by just a little you may not have to make any purchase at all.
But for each time you dress of the top of the front sights, shoot 20 or more rounds and see where they are grouping. Don't make changes based on one or two shots. Base teh changes based on a grouping. If the entire grouping is shooting low, then take some more off.