fish mouths

Now that I've got all that lovely tube lying on the floor of my garage, I couldn't resist seeing just how difficult it would be to cut my welding joint fish mouths.
Cautiously i cut off an 8" section (that's about £2.30! A whole pints worth!) and despite my best calculations and paper templates, got the first one completely wrong - i cut to a depth of half the diameter - which in theory would be correct if i was making it out of tube as thin as paper. My second attempt (as seen in photo) accounted for the wall thickness AND a weld bead - which means that the fish mouth isnt so deep (and requires far less filing) - ie its the external surfaces that I need to make flush, not the internal ones.
I reckon that the work of cutting and shaping the joints wont be too hard. The challenge is going to be protracting the profile to cut to; Theres a distinct lack of edges to reference to (and this practice one was a simple right angle!).
I suspect its going to be a job of tailor-making each joint with loving little file-and-fit matchups. One step at a time..... one step at a time.