The following are charts presented by other authors (see titles for specific URL information) comparing gain results for NVIS antennas at various heights. The chart from W4RNL (L.B. Cebik) has been reversed so that it is easier to compare to the chart from the Montana site.
Keep in mind that they are comparing different antennas (one wave length loop vs half wave dipole) but they both come to virtually the same conclusion. Simply that you should get about 4dbi gain at one tenth to one third wavelength above the ground. Remember that you need to subtract 2.4 from dbi to get dbd.
What all that really tells you is that if you run your NVIS at over ten feet and less than 1/4 wave length it produces virtually no difference in usable gain. Let me restate that. The total difference in gain between an NVIS at fifteen feet and one at 1/4 wave length above ground produce less than 3db difference in gain. That is LESS than 1/2 an "S" unit.
From http://www.cebik.com/wire/cb.html
From http://www.co.missoula.mt.us/acs/ACS/NVISpage1.htm
