This is a simple guide to ensure that a picture fits a mount well.
Firstly, lay your picture on a flat work surface and check with a set square to see if it is square. Measure both the parallel and verticle sides and take the shortest measurement from both as your picture size.
To arrive at a final window size you will then need to subtract 5 mills from both the top and sides.
Always work outwards from the image unless a picture is to fit directly in a frame without a mount. The size of the frame is the sum of the size of the picture plus the width of its surrounding mount card which typically by convention for an average-sized artwork is between 45 mils and 50 mils.
Finally, add an extra 5 mil to the vertical measurement of the surrounding mount card and you will arrive at the overall size of the framed picture excluding the width of the moulding chosen.