I need to space out rings in a grid to create a circuit layout, similar in nature to...
http://shrimping.it/blog/wp-content/upl ... randed.png
...but the measurements have to be exact as I'll be using the vector art to drive a laser cutter.

Consequently I've been wondering what I can use to space out items exactly (the ring centres should be 2.54mm apart).

I've tried Edit > Clone > Create Tiled Clones but it seems impossible to use shift to specify actual offsets (all offsets are bizarrely in proportion of tile size, and I don't know how to change the tile dimension to be which is bigger than the rings).

Tried to use the 'Remove Overlaps' feature of the Align and Distribute dialog but this doesn't seem to have predictable behaviour either.

With either of these approaches, when I check the results my 2.54mm high reference line doesn't match the gap between the distributed rings.

Can anyone tell me what is the correct way to distribute items using numbers and units, so that they are accurately positioned? The grid I need is 19 x 34 rings so it's infeasible to do it manually.

Welcome to InkscapeForum!

Yes, since the spacing in Tiled Clones dialog uses percentage, you'd have to do some math to figure out what percentage of the object's width and height equals 2.54 mm. So if I'm recalling the proper math formula, it would be: (2.54 mm/object width in mm) x 100 = value for X, and (2.54 mm/object height in mm) x 100 = value for Y (where / means divided by, and x means times). Although someone should confirm that's the proper formula, since it's been a long time since I've used it.

Or instead, you could use a grid, and align the objects using snapping to the grid. That could be tricky as well, since....well, maybe not. At first I was thinking that you can't snap the center of the object to the grid. But you can. So the hardest part would be making the 2.54 mm grid, and duplicating and moving the objects one by one. Snapping could be tricky as well, but as long as you have the snap bar set up properly, it should be fairly easy.

try the rows and columns dialog and enter the exact amount of x and y spacing .... in my Inkscape version with up to 4 decimal places of precision!

"rows and columns dialog"??? I think must only be available in your version?

Oops, no, I'm wrong! I honestly have never even noticed that at the bottom of the Objects menu! http://tavmjong.free.fr/INKSCAPE/MANUAL ... Align-Grid