I m not the gutter man but i ve seen how the guys did this on the house i built it also had a bay.
Guttering around bay window.
The reason that they don t make bay miters for plastic gutters is because they really don t sell very many.
The average seamless gutter job installed in the usa is only 640 00.
Gutter angles for bay window.
Floplast mini gutter is also commonly used on small lean to roofs and around bay windows where the 135 angle that is rarely found in small gutter systems comes into its own.
Seamless gutter around a bay window cut the gutter on the angle 22 5 degrees and screw the gutter to a miter strip corner then seal it screw it and install one gutter at a time on the house.
Don t think i have ever seen a bay window with gutters.