Not sure if any of the following information will be helpful, as it usefulness may be related to ease of use, but here's a few things that may help you along:
Rulers:
• The ArtRage Rulers have a pixel measurement indicator. If you place two rulers on the canvas (one horizontal; one vertical) with their "0" points along the edges of the canvas, you'll be able to approximately determine where to place the stencil.
○ Holding Shift while move the rulers will make them Stick moving exactly horizontal or vertical, so you can slide them over the canvas easily once you've positioned thier "0" points.
Grid:
The Grid feature let's you overlay the canvas with a grid that defaults it's 0 point to upper left of the canvas. It only allows you to go as low as 5 pixel spacing, however it still might be useful if you're looking to space things out. The grid also allows for snapping.
Paint Symmetry:
I know, I know... What's this have to do with measurements, right? Bare with me though. You see, showing the Paint Symmetry grid puts a horizontal and vertical grid (default) over you canvas, which allows you paint a mirrored mark on either side. Our interest in this, however, is that we can also numerically input where we want the center of our symmetry to be, which also means we can use the grid to determine exactly where a specific point on the canvas is. (for a visual of how to do this, please see this video i made, where I used it to create an image map for web use:
VIDEO LINK