Prompt user to pick point on circle with given radius

Hi!

I'm searching for a solution to achieve the following:

I need my users to pick a point p, enter a distance r and a direction v. A new point should be created r units away in vectorial direction. Therefore, the new point is located on an imaginary circle with radius r and center p.

Currently, the user picks the first point and enters a distance. Then, he has to pick another point (Editor.GetPoint with UseBasePoint = true), I calculate the vector between both points, create a new point (with location = BasePoint.location) and translate it d units in vectorial direction. This works of course, but the problem is, that the user doesn't see directly, where his new point is located, since he may pick a point closer to or farther away then r units.

Naively spoken, I want to restrict the line between BasePoint and new point while prompting the user to a fixed length. Is there any way to achieve this? Native calls to ObjectARX-methods would be also okay.

Thank you!

Image may be NSFW.
Clik here to view.Image may be NSFW.
Clik here to view.Image may be NSFW.
Clik here to view.Image may be NSFW.
Clik here to view.Image may be NSFW.
Clik here to view.

Image may be NSFW.
Clik here to view.