how can I find 3 resting points?

I have a part created by someone else.  It is a lens from a pair of glasses. I would like to create an inspection document that could be used by a QC person with a set of calipers (and maybe something to measure angles).  The inspector would set the lens on a table and take a few measurements while the lens is in contact with the table. 

You will see that the planes are not immediately helpful because the front plane can not be used as the surface for the three resting points of the lens. If I simply offset it downwards toward the inside of the lens it does not hit three resting points on the inside surface of the lens.  Is there a way to determine what three points would make up the resting plane?

I have been able to kind of fake it by offsetting a second plane from the front plane where I can adjust the offset so that the offset plane is close to intersecting two of three resting points on the backside of the lens.  I am estimating those two points.

Then to define a plane that goes through those two points and a third point I can sketch a line approximately where I think the two points intersect the plane.

And then I can rotate a new plane along the sketch line until it gets close to a third resting point. I am eyeballing this and not sure if I am really hitting the exact right three points.

I don't have the physical piece so I am estimating the location of the three points where the lens contacts the resting surface.  I am trying to find the resting plane so that I can bring the lens in to a drawing on that plane and then dimension the drawing for a QC person to use to inspect the lenses. In reality I have what I need to create a dimension document where I either use the dimension tools to get exact dimension or I sketch lines and draw in the dimensions to give something useful for the QC inspector.  I'll be faking it but it will be close enough for the needed inspection document.

But I am curious if there are obviously better ways to do this?  Are there tools in solidworks that I am overlooking that could find the three points that make the resting plane?

Message was edited by: Steve Krause