It's very easy to remember. Morale can range from -3 to 3, and luck from 0 to 3.

3.... 3/24 = 12.5%
2.... 2/24 = 8.3%
1.... 1/24 = 4.2%
0.... 0%
-1... 2/24 = 8.3%
-2... 4/24 = 16.7%
-3... 6/24 = 25%

As you can see from the table, it is highly undesirable to have negative morale, as your troops may skip turns one after another.

I would clarify that morale works differently in battles with heroes and neutral creatures.
In essence, morale is as follows:
3.... 3/24 = 12.5%
2.... 2/24 = 8.3%
1.... 1/24 = 4.2%
0.... 0%
-1... 2/24 = 8.3%
-2... 4/24 = 16.7%
-3... 6/24 = 25%
but there is one caveat that I noticed: in battles with heroes, the morale of the attacking hero is doubled and becomes as follows (only for the attacking hero!):
3.... 3/24 = 25%
2.... 2/24 = 16.7%
1.... 1/24 = 8.3%
0.... 0%
-1... 2/24 = 8.3%
-2... 4/24 = 16.7%
-3... 6/24 = 25%