Inkscape: How to create a tiled clones using shift in millimeters, not percent

I'm trying to create a triangular grid in Inkscape. I'm going to create a slanted line then clone it along a direction to obtain a fence, then clone the fence at 60 and 120 degrees to ob开发者_开发知识库tain a grid at, for example, 5-mm steps.

The problem is that creating a clone is specified by shifts expressed in percentages of the original object's dimensions. Since the original line is slanted, it's dimensions are expressed in ugly fractions of mm. So, to specify a shift in percent that would correspond to a 5-mm step, I have to calculate even uglier fraction. A small inaccuracy of the percentage can lead to the cumulative shift error, which may result in distant clones not being on the regular 5-mm grid.

How can I specify the shift of the clones using millimeters, not percent?

It seems that this question has an answer typical to free software: this issue has been highlighted in 2005, and hasn't been fixed (or, actually, implemented) for almost six years - see the bugtracker: https://bugs.launchpad.net/inkscape/+bug/170835