One issue I've found with the trace slope is that the direction is adjustable (can be down or up) and ORD isn't smart enough to change the direction of the line.

This causes two issues:

1) One of your flow directions is going to be wrong, depending on how you set up your flow arrow.

2) Additionally, the "low point" can be defined in the symbology, but if you select the "up" option, then the "low point" is actually placed at the high point, rather than the low point!