Cutting custom miter boxes (for varying angles) for guttering project

Tools you should probably have:
Sliding T bevel
Pen and blank stiff cardstock
Compass/Pro tractor
Compound Miter Saw
Decent pair of tin snips or aviation tool
Drill with appropriate bit size no bigger in diameter to the shaft of the rivets.
Riveting Tool with decent sized Rivets
Seam sealant

So firstly if you weren't a pro (like moi), unless you had access to seamless guttering equipment, which you might not have, its probably wise to build custom boxes when the angles weren't 90 degree corners or typical fits around house corners.  But generally you'd probably want to check even if house corners appear to be perpendicular that they really are, or in other words make sure the corners are 90 degrees before investing say in a prefab'd miter box.

So here's a procedure, that I've used for custom miter boxes.

Note cutting guttering on the electric saw.  Thin metals may tend catch on your saw if you keep the saw running even after the cut has been made.  Usually I make the cut, then turn off the saw while the blade is still in its full cutting position, and then only lift the saw blade back to its normally retracted position after the blade has come to a full stop.  

1.  Use the Sliding T bevel to measure, your inside corner.   I'd assume you know how to measure the angle of corner with the t bevel, or at I'd imagine someone probably has how to post if you weren't sure.

2.  Lay the Sliding T bevel, flat on your card stock, and trace the angle on the card stock.

3.  Measure the angle using your handy protractor.

4.  A bit of computation.  Now, if this an inside miter, the angle will probably exceed 90 degrees but be less then 180 degrees.  Regardless, you will divide this angle by two, and save this resultant angle.  If this resultant angle is greater then 45 degrees, add 90 degrees and subtract this result from 180 degrees, and record this new resultant angle  (This is using a right triangles formulation by the way).  If the original resultant angle is less then 45 degrees record and keep this.  Regardless you should have a final resultant angle that is less 45 degrees.  This resultant angle will determine the angle setting to be used on your compound miter saw.

5.  For inside corner boxes, you the outside leading edge will be the shorter relative to the gutters backside to be used and saved when you make your cut.  While the for outside corner boxes, the outside leading edge will be the longer then the backside.

6.  I make cuts flipping the gutters upside down, and setting the resultant angle to the left side of the 0 mark.  Finally cut end of your box at the zero degree setting on miter saw.  Then cut the mirror resultant angle of your previous work which is the resultant angle set to right of your zero mark.  Again after cutting this, reset the miter saw to zero, and cut off your desired miter box length.  I usually like each box side to be approximately to be a decent enough to work with decent enough room for the space you need.

7.  Both pieces should have a nice mirror symmetry join if your cuts are done correctly.  You can check to see that the fit is flush to your desired corner likewise.

8.  After checking for fit,  I cut some tin snips to fit along the seam of the two miter pieces.  I make rivet weld joins along the seam of the inside bottom of the gutter, and one on the backside, and then finally along the outer leading edge at the top with a small strip that generally probably won't be visible in most cases. 
I recommend drilling one hole at a time and then riveting each join point at a time.  Until you formally join your edges flush between the two box pieces for a given side, you hadn't need worry about edges being flush.  More critically, may help having an extra pair of hands to keep the pieces held flush, when two edges need to be join riveted.

9.  Seam seal your results if it looks good.