Thank you Anthony.
I'll try out the new numbers.
(I previously used the MoI distance script on the two close points, for Y and Z difference, and tried to reverse calculate, and got two multidecimal numbers, but must have messed up the math...)
Also it is possible to drag the spline "tangent" control point, over a tiny bit to meet the arc radius...

Currently trying to script a couple of 3 point arcs, for an intersection. I think "ring circle" script has the techniques to assign a frame, and script enter the
3 points, or two points and an angle.

I'm using this code:
var arcfactory1 = moi.command.createFactory( 'arccenter' );
// angle of +/- _ degrees.
var angleinput = 180.0 ;
arcfactory1.setInput( 0, pt_leArccenter );
arcfactory1.setInput( 1, pt_leArcStart );
arcfactory1.setInput( 3, angleinput );
arcfactory1.update();
arcfactory1.commit();
(repeat for second arc.)

It does not draw anything on the screen. I think the "frame" has to be set?

Next step to create the tangent point, maybe something like this: ?

var myArcList = moi.geometryDatabase.createObjectList();
myArcList.addObject( arcfactory1 );
myArcList.addObject( arcfactory2 );
var tanPtfactory = moi.command.createFactory( 'intersect' )'
tanPtfactory.setInput( 0, myArcList );
tanPtfactory.commit();
?

Still have to study burrman's method...