Rounded workpiece help...

With the 1013 only doing raster carving (back and forth like a printer) , there is not an uncomplicated way around it. You can however make your lettering a little deeper than you really want when you design your work piece, say by .7 mm, then when you are jogging the axis around in the set up on the machine, when you get the setting for the Z Depth instead of moving it down to just touching the work piece, leave it above the piece by maybe .5 mm . Jog the X down to touch the work piece and the move it back up by the .5 mm. That should result in you carving air everywhere except your lettering. You can also play around with the numbers I mentioned above by trying it on a small scrap to see which ones work the best.

I've done some work with round pieces, mine are table tops instead of bar stools but the results would be similar. First thing I'd recommend is mounting your round piece to a rectangular scrap piece, and then use the clamps on the rectangular piece. I've yet to get a round piece to stay clamped and not get ruined during the carving. Second I'd recommend that you use the starting position on your ipicture software to start in the very center of the piece, this allows you to find the center of the piece and based upon your design you can use the x dimensions to match the diameter of your stool seat. I carve 15" diameter table tops, and mounted this way they will go through the 1013 just fine.....even though your x dimension is limited to a slightly less amount. Since I start at the middle, I can carve to the max dimension of the x available and the table chooses your location based upon the very middle of the wooden circular piece.

I made the picture below on MS Publisher as a quick example of what I'd do in your application, the circle was left on as a visual reference but I'd take it off prior to carving or ipicture would try to carve that line also. I'd also crop it to just outside the wording in the final picture, so that I could control the size a little better on the cut piece. So if I had a 12" diameter round piece, and wanted the wording with some spare space on the outside, I'd set the x/y dimensions at 10" (as an example). I believe this would give you almost a radius of 5" to the top of the lettering in my example picture, if the cropped picture was right outside the text.

Bob is correct with setting the z dimension to insure that you are cutting air instead of wood, and since your scrap base piece will raise the barstool up if the bit goes beyond the round shape it will only be carving air and won't hurt anything. He's also correct with the trial and error, take a scrap piece of regular wood and carve it on fast to see if it matches your desired finished product. Once you have what you want and you see how the diameter would match up with your wording, you can then begin carving a barstool for real.