Mark, you can put in 14 or 15. It really doesn't matter as the spine dif with the calc is only a pound or so and less in real life. If you have a scale, you can weigh the shafts, add 15 gr for your finish and divide by the shaft length.
Any wood will vary in weight between shafts. Spruce has less variation than most, but Fir and POC will vary 100 grains or more within a given spine range and the higher the spine the higher the average weight. Then there are diameter differences, too.