If the center point of the radius lies outside the walls of your shop or if there are obstructions in the way, you have to resort to something like what Scott Grove showed.What's wrong with a pin, piece of string and a pencil. The pin is tapped into the centre point and the string wrapped around the pencil at the desired radious.
WOW!If you know the radius that you want and the distance between the outside points on the arc, here is how you calculate the middle point location.
C = radius
L = length between end points on the arc
Run a line from the center point of the circle through a point that bisects L, That line will intersect the third point on the arc.
To simplify the math, use A as 1/2 L, C as the radius and B as the line from the center to the cord line.
A² + B² = C ²
B² = C² - A²
B = The distance from the line to the arc is C - B
Radius = 20
Length between points = 6
From center point of circle to line is:
20² - 6² = 400 - 36 = 364
View attachment 398114
Distance from center point of line to arc is 20 - 19.078 = .922 Ft = 11.064 inches