Had this problem myself, but got round it pretty well now. I use ESTmep just for ventilation, so I'm not sure if it'll work for you the same way.

I use the "snap polar" option on the toolbar most of the time, which allows you to snap to 15 degree angles, this covers a lot of problems. It's not always going to be at 15 degree increments though. If you go off at an angle that isn't at a 15 degree increment, turn of "absolute polar" on the toolbar. This allows you to get a "relative polar" by slowly moving the cursor away from the last item (still requires either snap ortho or snap polar to be selected on the toolbar).

If you have items that don't like going off at a non exact angle (i.e. a 45 degree bend won't appear if you've drawn 44 degrees!), then there's often a tolerance property you can set by editing the item. This will allow a bit of movement either way, much in the same way as if it were being installed on site.

Hope this helps.