In the screenshot below, I need to measure W1 and W2. The polygon is shown in green. There are two point features which lie within the polygon shown in dark green. I would like the distance from the points to the polygon boundary measured in a direction perpendicular to the linear feature shown in dark red. I can construct perpendiculars using the IConstructPoint.ConstructPerpendicular method. Is the easiest way to find W1 and W2 to construct perpendicular lines at the points shown and then clip those lines with the polygon?

That would be a valid option.

However, if the line is in the center of the buffer then a single IPolyline:QueryPointAndDistance would do to get half the distance you are looking for. Or determine the distance to the boundary (IProximityOperator:ReturnDistance) and multiply by 2. However, if the polygon shape is not as predictable as shown, your option is much better.