BBX SrfSplit Tolerance

I am trimming a number of trangular faces using an extruded closed curve using the Grasshopper BBX and SrfSplit tools. Once split the surfaces outside the extrusion are culled.

Every so often a surface is not split. This is when only a small corner (say 10%) is on the cut line. It is not an error in the cull as the surface is never split.

It would seem to be a tolerance issue. Do I set this in Grasshopper or Rhino and how? I've adjusted the tolerance values in Rhino but it doesn't seem to have made a difference.

Any ideas much appreciated.

Dean