Btw. if you have only 100mile.. that is not so much. So the question
rather is: What accuracy to you need? [1 m or 100m?] Unfortunately I
forgot how much distortion you would get for 100miles in UTM (which
depends of course on the latitude).
btw. a tip: along the touching meridian and in equator direction one
degree is approximately 111km (derived for a sphere of radius ?6373km?).
This relationship could be helpful for conversions.