you need putonlayer.rb and layerchange.rb plugins.
groups+components+putonlayer+layerchange = perfect control layers. 😎

Do you mean that when trying to draw a line between two points, the line itself is not created or there is no face created when the line is drawn?
There is a glitch in SU when it comes to very small models; under about 1/16" it won't create faces properly. The general workaround is to draw your geometry in a bigger scale (in metric it's easy, say 10 times bigger) and after you finished, scale your model back to the desired size.

it is simply a case of writing it into the VCB, so for your line you would write 3' 4 15/16"

hope that helps