Fitting a mitre corner to architraves (45 deg or otherwise) on site, couldn't be easier. When I was joinering we had little in the way of mitre saws, electronic angle finders and the like, we simply had skill.
To bisect the angle i.e. find the correct line(s) of cut/plane, simply hold the leg of your architrave up to the door post, a couple of stitch pins/finishing nails to hold in place at whatever relief you want showing at the post and and architrave edges, mark the top width of the architrave (at the header location) both sides, onto the post/wall. Take this leg away and offer up your header piece of architrave, same relief as the leg showed and mark the intersecting lines, again on post and wall. You will now have two intersecting lines defining both the inner and outer edges of both, connect the outer highest leg point intersection with the inner header intersection point and you now have the correct angle. Offer back up the leg (still with stitching pins sticking out the back surface) to the position it was in to begin the marking, and transfer both the the outer and inner points from the post/wall to the architrave leg and proceed to join them up with a line on the face of the architrave. This is the bit that challenges, if your architraves have a radius or chamfer on the edge you must look down on the face perpendicular, and see the transferred marks at the same time, draw these up the sides of the architrave very carefully ensuring they follow the curves/chamfers at a good representation of correct flow, when these lines reach the face of the architrave join them up across it to give your cutting/planing line, which might not be a precise 45 deg, but it's correct note - no angle tools used, no measuring and the reason for developing the marks to the face is to allow the cut to to be down from the face and the saw break out to be on the hidden reverse face