Help with parameter / radius

[srushton01]

Hi, Can anyone help? 

Below are two screenshots of a seat I've modelled (upright position).  The first picture shows the radius at 652mm and a measurement of 40mm.  The 40mm dim needs to always remain, however when I flex the width of the seat (as shown in the second picture), the radius stays the same and the 40mm becomes 61mm therefore making the overall depth of the seat longer.  I need the radius to adapt to the new width, but I don't know if this can be done or how?

Thanks.

[RDAOU]

@srushton01 

Trigonometry ... If you have the width of the seat (W) and the Height of the Arc (a) given known values...then you can calculate R using Pythagoras

Watch GIF Below

[barthbradley]

Revit Formulas for "everyday" usage - Revit Forum

[ToanDN · Accepted solution]

Attached is another approach to constrain the arc without using a formula, but the built-in tangent locks.  Not saying it is better or worse, but it can be done, at least for what you show.