Diane, the max is 820nm -- that is where the sensitivity drops below 50%. The minimum depends on the filter, with the one I use I get the range from 710nm to 820nm (between the 50% points for filter and film). Center of the sensitivity is then 765nm, so that is the wavelength should be corrected for.

In addition to the filter, the correction will depend on the lens used. I found that without correction the results were far sharper using an APO-Lanthar than a Symmar or even a Zeiss Planar (at that time I had a 210mm APO-Lantar, a 135 Planar, and a 150 Symmar). 0.25% of 210mm is only about 0.5mm (not 5!), which isn't really all that much and might be difficult to do precisely with most cameras. For 150mm the correction would be less than 0.4mm, which I didn't even think of attempting.

It turns out that it's impossible to make general statements about how much the correction should be. It depends on the (general type of) lens construction, the corrections made, the type of corrections made and so on.