What is the proper and accurate way to measure space for window screens?
Don't get to hung up on the exact size the piece of screen needs to be. simply measure the width and height of the frame and buy a piece of screen large enough. Normally it comes in standard widths and cut to length.  Once you lay and stretch the screen  over your frame, install the spline on one end, stretch, install sides and opposite end. An inexpensive spline roller comes in handy here. Use some sharp scissors, utility knife or shears and trim off the excess close to the spline. Done!