AutoCAD Inventor :: Sketch Flipping When Going More Than 180 Degrees

Here's the situation:

I have a tube, approximately 16" O.D., 14" high, and 1/8" thick. This is all a single part.

I'm trying to cut 3.5" wide and 4" tall slots around the top. The number of slots can be 1-6.

I'm driving the size and slots from a spreadsheet.

The orientation of the slots can have the first slot anywhere from 0 to 360 degrees. (FWIW, the spreadsheet redefines 0 and .0001 and 360 and 359.99).

Anyhow, as long as the first slot is between 0-180, it works just great. Once you pass 180, however, the verticals of the sketch flip so it's barely cutting the notch at all.

To make the cuts i've created a sketch off the top of the cylinder and drawn a construction line at 0 and another and put a driven dimension between them to set the correct position of the angle. I think create a workplane off that line tanget it to it's end point. At that point, draw sketch, extrude, etc.

I've also create a workplan off one of the origin planes and the origin Y axis that's perpendicular to the notch (angle driven by spreadsheet and equation in workplanes angle offset). Offset a 2nd plane tangent to the O.D. of the tube, created a sketch, extrude etc.

In both instances, when the angle crosses 180 degrees, the sketch flips on me.

What can I do to keep the sketch from flipping?