[Gregstrom]

I think PhilD's formula could come out as:
40 -( 40 * ( ( ((40-Natural Prot) /40)) * ( (40-Armour Prot)/40) ))

For 5 base and 10 armour (as per chrispedersen's example), that's

40 - ( 40 * ( (35/40) * (30/40) ) ) = 13.75

After making an Excel spreadsheet do the calculating:

5 base and 14 armour is 17.25

7 base and 14 armour is 18.55

20 base and 0 armour is 20

0 base and 20 armour is also 20


This seems pretty close to values in the game.