One of the difficulties needs to be labeled "average". This makes it obvious what the default difficulty is unless there is a reason to make it more or less difficult than average.
Here are the success chances of various characters attemting skill checks of their level at the varying difficuties.
The best person in the world has a 70-85% chance at succeeding at low difficulty task. This percentage general goes up with level. He has a 60-70% chance at succeeding at a high difficulty task, again generally rising with level. He has a 55-60% chance of succeeding at a severe difficulty task, with no correlation with level. He has a 40-45% chance of succeeding at an extreme difficulty task, again with no correlation.
A moderatle skilled person has a 55-60% chance at succeeding at low difficulty task. This percentage general goes down with level. He has a 40-50% chance at succeeding at a high difficulty task, again generally dropping with level. He has a 25-45% chance of succeeding at a severe difficulty task, again generally dropping with level. He has a 10-30% chance of succeeding at an extreme difficulty task, again generally dropping with level.
A person who has totally neglected a skill has a 20-40% chance at succeeding at low difficulty task. This percentage goes down with level. He has a 5-30% chance at succeeding at a high difficulty task, again dropping with level. He has a 5-25% chance of succeeding at a severe difficulty task, again dropping with level. He has a 5-10% chance of succeeding at an extreme difficulty task, and is incapable of achieving a critical success past level 5.
Stats for these definitions:
The best in the world starts with an 18 stat and puts all possible stat increases into that stat. He has a potent item in that stat. He advances to expert, master, and legendary at the earliest opportunity. He has the highest item bonus available at the earliest opportunity.
The moderately skilled person starts with a 14 stat and advances to an 18 by 15th level. He advances to expert by level 10. He gains an item bonus up to +3 over 18 levels.
The neglected person starts with a 10 stat and advances to a 12 by level 15. He is untrained and does not become trained. He gains a item bonus up to +2 over 16 levels.