My design rule specifies a trapezium with parallel sides
horizontal and other sides sloping inward at rate of 1:20 to
vertical.

This is a perfectly reasonable rule and I can sketch the trapezium with sufficient precision by specifying the slope as 2 deg 87 min.

However I then want to rotate the trapezium until one sloping side is Vertical.

Because 1:20 is not an integral number of degrees or radians, I can rotate to nearly vertical but not to vertical. The line's property manager shows the x ordinates of the two ends of the line as being slightly different at 7 decimal places.

How can I get the line vertical?

This is a perfectly reasonable rule and I can sketch the trapezium with sufficient precision by specifying the slope as 2 deg 87 min.

However I then want to rotate the trapezium until one sloping side is Vertical.

Because 1:20 is not an integral number of degrees or radians, I can rotate to nearly vertical but not to vertical. The line's property manager shows the x ordinates of the two ends of the line as being slightly different at 7 decimal places.

How can I get the line vertical?

The points I have taken are:-

1) Go back to the drawing board and define slopes with graphical construction.

2) Analyse the consequences of making a relation or constraint.