A better flatbed will fix this problem. Instead of scanning at low numbers and then using interpolation to get your frame size, you need to scan at higher resolutions, 9600 being a good starting point. If your flatbed scanner cannot scan at this resolution, it's time for a new scanner.
Also, using Photoshop is way too tedious. There are software solutions that automate this process. Cine-to-Vid comes to mind.