Also a quick note that may help you with some of the constructions that you're doing  if you make 2 construction lines cross each other at a single point there will be a "normal" snap for the line that is perpendicular to both of them.
So for example 2 lines like this that are not flat to any world plane:
Start the line command and drag 2 construction lines each of which starts at that end and goes tangent to those lines:
Then you can place the first point onto that same spot, and for the second point there will be a "normal" snap at the shared perpendicular:
That may help you in some situations to get some element drawn where you need it for building a cutting plane or whatever, without needing to set a cplane for that situation.
 Michael
