4x8 battle board question

If I have a 4x8 foot battle board, my calculations say that to scale for 38mm gaming, the 8 foot span should be calculated as follows:

real scale - 1 mile equals 5280 feet
28mm scale - 1 inch equals 6 feet
An 8 foot long table is 96 inches
96x6=576 feet at 28mm scale

I do not really want to see players moving across the map at break neck speed. If I throttled the speed some, how many inches should I allow them to walk at a time as i adjust/change things they may or may not see or hear?

JSM