I suspect this won't address your needs, but the base scaling of the commercial cartographic programs might give you a starting point:

Fractal Mapper (TM) 8: 1 foot = 50 Pixels (can be made flexible by settings in default.xml)
Dundjinni (TM): 1 foot = 40 Pixels
Campaign Cartographer (TM) 3 Dungeon Symbols: 1 foot = 100 Pixels
Campaign Cartographer 3 Other Symbols: 1 foot = 40 Pixels