define a point on a circle

Hello, 
I working on drawing of a stair. I want to define a dimension from a point on a line to a point on a circle's periphery.
The line is tangent to the circle.

I tried to divide the measurements in two, one for the line and the other for the circle. the problem is that I can't define the measurement on the circle.

I'm working in drawing.

Hope my question is clear.     

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