NavList:

But at the pole a minute of LONGITUDE is ZERO feet long since all meridians come together at the pole. The length of a minute of longitude at any latitude can be considered to be one nautical mile times the cosine of the latitude. At the equator the cosine of zero degrees of latitude is 1 times one NM equals 1 NM. At the pole the cosine of 90 degrees is zero so the length on one minute of longitude is also zero. At 60 degrees of latitude the cosine is 0.5 so the length of one minute is 1/2 NM. At 39° the cosine is 0.777 so the minute of longitude is 0.777 NM (4,722 feet) and at 40° the cosine is 0.766 so one minute of longitude is 0.776 NM (4,654 feet) 67 feet shorter. 

If you are drawing meridians one degree apart then the difference is 60 times larger 4,028 feet, 0.66 NM. In celestial navigation such small inaccuracies disappear into the noise of the observation. Since your AP should be within 30 minutes of longitude of your position the difference is actually only one-half as big, 0.33 NM. Statistical studies i posted before on Navlist showed that for very experienced navigators the expected errors are about 1.5 NM, almost five times any error introduced by the difference in the spacing of the meridians on your plotting sheet. So don't worry about, use either the 39° or the 40° spacing on your plotting sheet it will make no discernible difference in you fixes.

gl