This is why the OP’s question is complicated as it depends on the pistol, depth of the optic cut, height of the specific optic, and thickness of any adapters which may be required. The backup irons should be the minimum height necessary to clear the body of the optic which will put them somewhere north of factory sights in height, but lower than true “suppressor height” or 1/3 co-witness sights.You have a small optics window to deal with and the open sights just eat up critical real estate.
I run a Holosun HS 407K X2 on my Gen 3 G-19 (no adapter), with a .125 deep optic cut, and use a black Wilson Combat U-notch Battlesight .245 height rear, and an Ameriglo Pro-Glo .220 height front. Irons are just visible above the body of the optic, but don’t take up more of the window than necessary. For reference, I believe factory G-19 sights are .175 high rear and .160 high front above the slide.
It took me a LOT of searching on the interwebs to come up with that combo since most manufacturers put out products that trend with suppressors or the ever-popular 1/3 co-witness beginning around .315, so I hope sharing this saves time for others.