how long and angle to cut rafters on 4/12 pitch 10'1" wide and were to put the birdmouth how long and deep I using 2x4 for rafters 2x6 at peck

I believe birds mouths are cut to the center of the rafter. The carpenter uses the framing square to calculate everything. Since I don't know how to do this would be my approach.

Temporarily nail a 2x4 to the top of the ridge beam, flush with one side of the beam. Hold your rafter up against the beam even with the top of the 2x4. The other end resting on the top plate of the wall. Hold another piece of scrap 2x4 along side the rafter and mark the angle to cut of this end of the rafter. Cut the rafter. Put the rafter back in place and temporarily tack in place or have an assistant hold. Place a scrap 2x4 on the wall top plate and mark the birds mouth.

The reason for tacking a 2x4 on the ridge beam is that the rafter, resting on the wall plate, is 1 1/2" above its final location. The 2x4 is just a convenient way of marking where 1 1/2" above the ridge beam is. When the 2x4 and the birds mouth are removed the rafter drops down 1 1/2" maintaining the same angle and distance.