relative step value

[malcooning]

currently the 'step' setting in the CustomBrush tool is linked to the size of the brush.
or in other words, the step is not absolute, but is relative to the size value.
this can bring a desired result in some cases, but many times I find that I want the size of the brush to change (i.e. controlled by pressure), yet the distances between the dots (brush marks) to remain fixed.
I think there should be a button that toggles between relative/non-relative modes.