Probably easiest to describe with an example.
Suppose you have 3 Atk 20s and combined magicite attack is 500.
First gives +20% of combined magicite attack to your party, so 100.
Second is +20%/2 to your party, so 50.
Third is +20%/4 to your party, so 25.
(Fourth would be /8, I guess rounded up, so 13)
For a total of +175 attack to party (would be 188 with 4).