This is my methodology: take the official batting rankings, and assign one penalty point per ranking place. So that first place is one penalty point, tenth place is ten, etc.
Tests have a weighting of 3, ODIs 2, and T20s 1. If a player is not ranked in the top 20 they score a maximum of 20 penalty points (subject to weighting). Thus the minimum possible is 6 and the maximum possible 120.
Some players
Steve Smith (3 Test, 30 ODI, 20 T20) = 53
Joe Root (6 Test, 34 ODI, 20 T20) = 60
Kane Williamson (6 Test, 8 ODI, 6 T20) = 20
Virat Kohli (42 Test, 4 ODI, 2 T20) = 48
Martin Guptill (60 Test, 18 ODI, 5 T20) = 83
AB de Villiers (21 Test, 2 ODI, 20 T20) = 43
Hashim Amla (12 Test, 6 ODI, 20 T20) = 38
David Warner (15 Test, 36 ODI, 12 T20) = 63