I print almost entirely on “round” Christmas bulbs.  To be sure the object is centered, I place the Egg-Bot on top of a sheet of printer paper, then use a sharpie to place a small dot of ink on the page.  As I stand over the bot, I align the edge of the bulb, visually, with that small dot.  As I manually rotate the ornament I can see “wobble”, and that tells me where and how far off my centering is.  I correct the placement, then check again.  When I can spin the bulb 360 deg. and not see much wobble I call it good and begin the plot.  It’s a technique used on well over 100 bulbs and is often good for such  projects.  Video of last years ornaments is here:  http://www.youtube.com/watch?v=cfv82nj9AMM You can see my notebook paper under the Egg-Bot, but the alignment dot is just out of the shot on the bottom edge of the video.  My biggest hurdle was with the origional clear pads… they didn’t work very well with the smooth glass.  A precision egg coupler solved most of those problems, and I highly recommend it if you don’t already have one.  Trial and error works best, and I’m sure you will find something that works well for you.