John, the best thing to do, which is easy with roll film, is to run a quick test to determine the best exposure. On a bright, sunny day, find a scene containing blacks, grays and whites. Make six or eight exposures of the same scene over a six or eight stop range. Try 1/4" 1/2" 1" 2" 4" 8" 16" 32". Somewhere in the set of exposures is the best one. More than likely you'll be able to tell by looking at the negative and finding the one with the most detail in shadow and highlight. That exposure will be your "magic number" or your "sunny 16" number. Adjust your exposure by the difference, in stops, between bright sun and ambient conditions. If you used your Nikon to make a meter reading when making the set of exposures, you can use it in the future to help you make the adjustments for ambient light.
Let's say the best exposure was 2" and the Nikon said 1/100" at f16 at ISO 100. If you make an exposure on a hazy day, and the Nikon says 1/50" at f16 at ISO 100, you'll need to make the pinhole exposure one stop longer (two times longer). So your pinhole exposure will be 4" If the day is overcast, and the Nikon says 1/25" at f16 at ISO 100, you'll need to make the pinhole exposure two stops longer (or four times longer), or an 8" exposure.
This is actually a simple system, if you understand the rudiments of photographic exposure. In practical use, you don't need to worry about reciprocity failure, since your initial test was done with such a long exposure.
This next comment is unecessary, but I'll make it anyway. I use an indicent light meter set to display Exposure Values, which makes the process of determining the difference between sunny 16 and ambient light even simpler (except for the learning about exposure values in the first place, which is why the comment is unecessary).
P.S. Ric, great to see you on APUG in addition to f295.