If I turn my grip on the lathe then drill a recess with a dremel, I have no problem making even grips where the recess is centered. ’ll ream the grip, then dry fit to the blank where the seat is and mark the recess on the grip.

I don’t really have a problem doing this as I have gotten pretty precise using a dremel. However, I have been trying the flex coat method of altering spade bits and drilling a recess in individual cork rings as I like the thought of how quickly I can get a perfect recess and since I don’t venture out often on reel seats, I could save time and can make a few rings for future builds.

The issue I am having is that every time I shape the grips, one wall of the recess gets super thin before the other. I took a photo and attached so you guys could see. What am I doing wrong?

For clarification, I have a mandrel slightly smaller than blank butt diameter, I drill recess in cork on drill press, glue rings together on mandrel, shape to even cylinder close to diameter of the largest part of the finished grip, and then shape to final form.

You are doing nothing wrong.
Holes in cork/rubberized cork are NOT concentric. When you use the 1/4" hole as a guide you are guaranteed to get what you got.
That's why the first thing you have to do is to make the grip round and concentric whether you build on a mandrel or on the blank.
You have to knock-down the high spots with a Stanley Surform tool.
Use a light touch and ONLY hit the high spots - 3:00-4:00 o'clock in your picture.
Herb
CTS Rep.

I would say that the hole in the cork rings aren't centered. I made a jig based on the jig in the video I am going to link. It made a big difference in the few grips that I've built.

I've also used it with a Forstner bit to bore a recess in cork ring without a center hole for use as a butt cap.

[www.youtube.com]

I do as David does and utilize the Forster bit with great success.