Nalla,
As Dave has stated lay it out on some sheetmetal.
I would cut a straight edge with a Vee ( 5 mm high) at 55 degrees included angle (2 x 27.5 degrees) that way you can use it for prepping and also for completed set up of joint. (Just add the root gap)
Bit hard to explain without a diagram - hope you understand what I am trying to say,
Regards,
Shane