Some times you just can't draw what you want on the model itself or on a created plane near the model. This can be very frustrating.
In these cases, I often create the geometry that I need on the ground and then copy and paste bits of it (or the whole thing) to the part of the model where I need it. This is particularly useful for creating accurately sized small lines when the model is already big and you can't draw what you need in its proper position.
If you need the new geometry to be on something other than a horizontal plane, you can still create it horizontally and then rotate it into the correct orientation before copying and pasting it.
If all you need is a line and it's not horizontal, then create a plane with the line you want as one side. You can then rotate the plane and copy and paste just the line in the correct position.
If you have trouble attaching your new line or geometry where you want it (it can flop around all over the place if it isn't geometrically defined enough for Shape to understand what you want), then extrude the plane with the line and then copy and paste the whole solid where you want it. Then just delete the extra geometry.
Phew! It may take a few tries to get what you want, but it will almost always work.