Ok, so we're on the same page--
I'm assuming YES to this, but going to ask anyway--do you have a red pointer, AND can you display your laser's current coordiates, either on the machine display, or in the software?
-- I live and die by these coordinates...
I use the pointer to locate the top and bottom of whatever I'm engraving. Example, what I consider the top of a Yeti as being held in my machine is usually about 1/2" or so from 'zero-left', as found by the red dot. Then I find the bottom. Then I put guidelines at these points. Next, use the machine to slew the gantry (X) and rotate the part (Y) to find the location of an existing logo (like the Hydroflask logo f'rinstance). I find the top, bottom, left and right edges of it, write down the coordinates, then make a box on the screen to match them. Yes it's time consuming, but takes less time (and money) than do-overs!
Now everything you need to know is on the screen, and you can move whatever you want to engrave relative to your coordinates. Just make sure nothing extends above or below the working area. If it does, then you'll need come up with a new straight-up 'zero point'.
I brought up Hydroflasks because ironically, I'm setting up a job of those right now, and my customer changed her logo, added more text. Now I have to see if I can still engrave them without removing the paper strip they glue on around the bottom. I'll be using the red dot to find the top edge of the strip and the top of the bottle, and HOPE the new logo fits above the strip without having to move the logo up the bottle too far. If it ends up not looking right, I'll be pulling the strips off all the bottles first (which I'd rather not do!)
To set these up, I put the Hydroflask guy straight up, that's my zero-point, and the customer logo will be 180 around on the other side.
Finally-- you can 'test drive' your Cermark (or whatever yours is) by simply engraving it fast (say, 80% on your Epi), at very low power (4 or 5%) and at reduced resolution (like 200dpi, just to speed up the test process).
Find a power setting just high enough that it goes dark enough to see what you're engraving. You'll have an visual you can look at and measure to see if it's right. And it won't EVEN stick to the metal at those settings...