GCPnts_AbscissaPoint on either side of a curve

Hi,

I'm trying to compute the abscissa on either side of the parameter on a curve.
To do so, I've got a reversed and non-reversed instance of the curve.
To get a decent estimate of where the abscissa should lie on either side of the curve, by computing the intersection of a sphere, placed at the `from` parameter on the curve.

What strikes me is that even though I've reversed the curve, and have given good parameter estimates, that GCPnts_AbscissaPoint finds the same point on both the reversed and non-reversed point. Worse, also points are found that lie not at the given distance at all. Have a look at: http://jelleferinga.com/files/abscissa.png . You'll see some vertices that lie on the inner blue ring.
The vertex close to the red line represents the `from` parameter of GCPnts_AbscissaPoint. In three cases, the right point is found, but if you look at the top right vertices you see that the found point is really close to the `from` parameter. Could it be that GCPnts_AbscissaPoint is not handling seams well? Also, I'm curious how I can find the abscissa on either side of the curve!

Thanks in advance,

-jelle