Very High = 15
High = 10
Low = 5
Very Low = 0

is this right?

You have it back words. 0 is highest priority.

The reference value is a number between 0 and 65535 that indicated the mail exchangers (mx's) priority.

0 = Highest Priority
65535 = Lowest Priority

Depending on the number if MX's your using - create your own naming convention.

We use:

0 = Primary MX
10 = Secondary
20 = USP-HSP Primary
30 = USD-HSP Secondary

Keep it simple - and level across the board :)

moz.