I'm trying to fit trim on a square post. The trim goes around the base of the post where it meets a stone foundation. The trim is made from a fir 2x6 with the inside face (where it meets the post) beveled 20°, making the angle between the top face and the inside face 70°. I'd like to join the pieces with miter cuts at the corners.
The way I'm setting up the cut is to use an angle finder, similar to this one:
to first measure the angle between the sides of the post, and then to transfer the angle to my miter saw using the laser guides on the saw to adjust the angle on the saw until the laser runs down the guide line in the center of the tool. The blade is set plumb to the table and the trim pieces are clamped to the saw fence so that the inside face is flush against the fence (in the same position as the trim will sit against the post).
After cutting both side of a joint, the angle between the inside faces does not match the angle finder and the piece does not fit the post exactly (the angle is a little wide (about 1/8" over 14").
I've double checked everything in my setup, and I can't find any mistakes. I'm looking for suggestions on how to troubleshoot this, and also a sanity check on my method – am I right in thinking that a piece of trim being used in a manner similar to crown molding can be cut at the "obvious" angle (e.g., 45° for an accurate 90° corner) when it is held in the saw in the same position that it will be in when installed?