One thing that will help is to know that a basic arch is just a semi-circle, and a circle's curves are symmetrical.  In your arch, your first part of the curve goes "up 2, over 1" (that might be something like, up 16 units, over 8 units, I can't tell)
 
 That means your LAST part of the curve (at the top of the curve, where it starts to flatten out, needs to be the reverse of that:  Up 8, over 16.  That should make it look better already.
 
 Another alternative to above is you can start with a large solid block exactly the size of the texture of the arch, and start making horizontal splits/cuts when the arch texture requires that the geometry needs to start bending, keeping in mind that the cuts/distances should be symmetrical
 
 I have not used Trenchbroom which I assume is what you're using but here is what I mean:
 
 1. Block size of arch
 
https://i.imgur.com/jtyYfKy.png
 
 2. Split block where bend starts
 
https://i.imgur.com/knieCHf.png
 
 3. Clip away non-curved part
 
https://i.imgur.com/QZxlnsx.png
 
 4. Split arched part horizontally, absolutely as close to the "middle" of the 90-degree arc is, and clip it away.  The height and length of that clipped away part NEED to be the same distance. (Unless your arch is intentionally ellipse shaped)
 
https://i.imgur.com/McbpLYp.png
 https://i.imgur.com/leaGWCJ.png
 
 5. Subdivide (horizontally) the remaining blocks another time.  Again make sure they are perfectly mirrored and the cuts are symmetrical, you'll see what I mean by color coding in pic.  Then clip/vertex-edit them away
 
https://i.imgur.com/o1TYdP1.png
 https://i.imgur.com/lmmFaw1.png