I wondered about this. Is there a way to lock (like with SHIFT key or something) the tangent, perp., etc. before I click?

These first two are good examples from the top view.






I started with the triangle shape to get the proper angles.



I then copied the lines twice -z axis.



I added some guide lines and made a connecting circle with tangent (trimmed it later).



Then I made a couple of circles, rotated them to line up with the big triangle, and trimmed them.



...after adding the rest I noticed the errors and started over. Keep in mind, this was an incomplete first draft so it looks awful. I was able to make corrections and finish the shape.

Forgive my simple descriptions. I am a hobbyist and not familiar with the terminology of 3D programs. If I don't make sense, please let me know and I'll try to clarify.