NO, Sorry that's wrong.

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6/12 is NOT a 45 degree angle, 12/12 is..

I didn't read any more about the post, but it is flat wrong.

The correct way to get the 6/12 pitch, and the easiest way is to lay the rafter on the ground, and cut the peak end of the rafter. You can either precut the rafter tails, or cut them in place, but don't worry about that cut at first.

"THe internet method"

Lay the rafter on the ground so when you look left to right you are looking across the length of the board. The first mark will be made as follows.

Mark the top right corner of the board.

Using a carpenters square, lay the square so the board is inside the notch of the inside of the square. The long part of the square should touch the length of the board and the short edge the width.

Take the dimension of the width of the rafter. Given a 2x6, for example, the actual dimension of the 2x6 is 5 1/2". Take half of that dimension, or 2 3/4" and make a mark on the length of the rafter on the opposite side of the reference, but on the same end of the board, of course..

You'll now have a mark on the upper right corner of the rafter, and another mark on the bottom edge of the rafter 2 3/4" over.

This is the line to cut.

Place the rafter against the ridgeboard to determine the desired length with overhang of the rafter and mark the location of your cut.

You can also use trig. The 6/12 pitch is a tangent. The inverse tangent of a rise of 6 over a run of 12 is 24.09 degrees, not 45.

Given you have mastered the cut the first time, you might be able to use this method.

Placing the speedsquare on the pivot point, locate the common rafter 6 and pivot the square until the 6 mark lines up on the other edge of the board. Same mark, better tool if you know how to use it. The work great for cutting rafters, too, you just set the square over enough for your sawguide, hold it in place, and move the saw against the square for a perfect cut.

There is another method, but I'm outta here.