[bfblack]

Here is my version of a mortise jig that was made with a router table since I do not have a CNC machine. The jig is for a nominal 3/8" thick floating tenon and uses the same 5/8" OD guide bushing as the OP. I use a gauge block to align my fence parallel to the edge of the guide bushing slot. For 3/4" stock and 5/8" wide guide bushing slot, the fence should be 1/16" away from the edge of the guide bushing slot. You can repeatably achieve this offset to within a few thousands of an inch just by using your finger to feel the mismatch. I challenge you to achieve that repeatability if you are aligning the jig with a layout line on your stock.
I make my own floating tenons with a thickness to match the width of the as cut mortise. Remember that if you have a router bit sharpened, the diameter will be reduced slightly.
If you want to go down this path, here are some suggestions for improving the jig design:

1. Don't center the slot right-to-left if your router base has any asymmetry.
2. Have fence longer than the width of the jig base. This will allow clamping flexibility.
3. Make the jig base out of MDF or Melamine/MDF core. Both are more dimensionally stable.