I have recently had occasion to convert a few ships to Starmada from a background that used two separate layers of Defense. It seemed reasonable for me to model this by giving these ships two layers of Shields and rolling against each one. To my surprise, when I did the math for this, it turned out to be equivalent to the Expanded Shield Ratings rule. I provide the figures below to anyone who would find them a convenience.

Double Shield Ratings: Probability of Shield Penetrarion.

1, 1: 70%; 1, 2: 56%; 1, 3: 42%; 1, 4: 28%; 1, 5: 14%.

2, 2: 45%; 2, 3: 34%; 2, 4: 23%; 2, 5: 12%.

3, 3: 25%; 3, 4: 17%; 3, 5: 9%.

4, 4: 12%; 4, 5: 6%.

5, 5: 3%.

Expanded Shield Ratings

1: 84%, 2: 67%, 3: 50%, 4: 34%, 5: 17%, 6: 12%, 7: 6%, 8: 3%.

Double Shield Ratings: Expanded Shield Ratings Conversion

1, 1: 2; 1, 2: 3; 1, 3: 4; 1, 4: 4; 1, 5: 6.

2, 2: 3; 2, 3: 4; 2, 4: 5; 2, 5: 6.

3, 3: 5; 3, 4: 5; 3, 5: 7.

4, 4: 6; 4, 5: 7.

5, 5: 8.