Hoping this is not out of line, there is a Hand Held Calculator on the website link below that is very user friendly and can layout just about anything, including Intersecting TEE's at any angle, including the HOLE layout. You just input the dimensions it prompts you for, including Material Thickness and it will give you all the layout dimension's you need for the layout. Just draw a BASE line around the pipe or tube, then draw equally spaced vertical lines, (16 spaces for 12"),from the baseline per attached Manual page, mark off the lengths the Calculator gives you, join the marks around the pipe, and theirs your mitered piece at any angle you input . You don't need to know the math and its Dead Accurate. You can use the Layout Manual that comes with it like a computer monitor for visual instruction and direction, see attachments below. It will also layout Square to Rounds, Round Elbows, Cones, Stair Cases, handrails and much more. I used the Square to Round program to layout the Large Square to Round PHOTO Attached. It always reads dimensions in DECIMAL form 1st, then in FRACTIONAL form 2nd, to the closest 1/32",+/- 1/64". This is for the person that doesn't know his/her Decimal readouts. It also has Utility programs for Drill & Tap sizes, Metric Conversions, Pulley sizes, including RPM and Belt lengths and more. This is a hell of a tool for people that don't have any layout experience or know the math. The site shows all the pages in the manual. Be sure to read all the "NOTES" in the pages, it helps things make sense. This Calculator is a Real Tool, its not a gimmick or a toy. Its worth a look at.
http://www.sheetmetallayoutcalculator.com