[aprssig] Fill vs. wide?

[John Dvoracek]

I've searched and found no guidelines although such surely exist, but
what's the rule of thumb for when a digipeater graduates from a fill
digipeater to a wide-area digipeater?  Is there an antenna height
about ground-level criteria?  I sysop digipeaters that vary from 90
feet to 400 feet above ground.  There could be all sorts of other
criteria I know, but, please, KISS mode reply!

73-John