The standard should always be, "... if a batter is judged to have reached base solely because of a fielder's mistake, it is scored as a "hit on error,"
The only case where the above, easy and straightforward, standard should not apply is in the case of "extraordinary effort", when the fielder dives for a ball, maybe even catches it but is not able to make a play, you can judge that as hit.
Only in the upside down world of softball would someone try to actually take the extraordinary effort language and then make some completely illogical leap to "routine play" and to "routine based on the worst player who has ever played the game level of play" to justify hits vs errors.
Also. Anyone that hands you the ATEC Scoring guide for softball should be laughed upon. These two gems alone completely discount this guide:
g - Merely fielding a ball too slowly is not an error.
h - A mental error is not recorded as an error.
After they hand you that guide, you should kick them off your team and let them go form their own team full of mentally defective slow walking players. Just cause it is posted on the internet does not somehow make it magically authoritative.
There are much better scoring guides out there, the ones on the back of the book are better. The ncaa one is much better:
http://fs.ncaa.org/Docs/stats/Stats_Manuals/Baseball/baseball_softball_scorebook.pdf
The other one I like is the "lost in the sun". See scorers score this as hit on an extremely routine fly balls. That is not the point of the "lost in the sun". The ultimate guidance is ordinary effort.
ORDINARY EFFORT is the effort that a fielder of average skill at a position in that league or classification of leagues should exhibit on a play, with due consideration given to the condition of the field and weather conditions.
So a 10 yr old on a tough fly ball in the sun, one that most 10 yr olds would drop, go ahead score that a hit. An 18yr old, applying that same standard you look absolutely goofy. By 18s, for the sun to "cause an error" it had better be a rocket shot, right at them, coming out of the sun to the fielders. Otherwise most other 18s would adjust and make the play. Hence ordinary effort.