When choosing how to number priorities I usually use 3 levels:
1) Critical. These are show stoppers. Without these fixed, the whole system is a waste of money.
2) Workaround exists. The customer have to call you (the developer), they might have to buy new hardware or send a backup copy by airmail deluxe. But a workaround still exists.
3) Cosmetic. This is anything that the users can figure out the solution for by themselves.
These 3 levels are not enough so they have 3 priorities in turn. 1, 2 and 3.
All in all 9 levels.
It is so simple it seems to work.