Spiral Stair Geometry

I’ve just added a curved and spiral stair geometry calculator to blocklayer.com and need some advice on an adjustment (addition) to it.
Spiral Stair Calculator - Layout Circular and Spiral Stairs - Metric
For spiral stairs, geometrically, the inside run of each tread is very short with a small centre radius. (post?)
If you enter 40 for Inside Radius and 14 for Inside Run, you’ll see what I mean.

From what I’ve seen in drawings, you’d need to widen the treads.
ie: inside to weld or add strength, outside to accommodate newel posts as here http://www.extremehowto.com/xh/article.asp?article_id=60902

So you’d need to overlap the treads to keep the correct run, angles etc as calculated on the geometric drawing.
I have added (on a test version) the option to add to the inner and outer run of the treads.
The geometry of the stairs (diagram) remains the same, but the tread detail is adjusted to suit the added tread depth (can be different inside and outside).

So the treads would need to overlap to maintain correct geometry.
On the test version, there’s a checkbox to add widening and inside and outside textbox entries for the amounts.
The adjusted tread diagram is clearly marked as widened and a (detailed) note (alert) added explaining that the treads must overlap to maintain the selected geometry for total turn angle etc.
Does this sound like a good idea?
It wouldn’t be too hard to end up with an overcomplicated mess of input boxes and selections and I don’t want to make it too hard to use.

Thanks for any guidance here

Should be helpful for those so inclined!

Just a bit late for Ad.:D

By the way, why are they called spiral staircases when they are, in fact, a helix.?? Spirals are flat!

a helix is a spiral. not all spirals are helices. see wikipedia