That was the idea (if using 40 and 50 % as factors). But in hindsight, I don't think I like it very much
Coming Last or second to Last shouldn't really be much different scorewise, so your formula is better there. There should however always be a positive score for coming 2nd, so a little tweak may be in order.
The "perfect" formula for a 4 player game should give something like:
A = +30 points
B = + 5 points
C = -15 points
D = -20 points
And for 5 players:
A = +35 points
B = +15 points
C = - 5 points
D = -20 points
E = -25 points
Now thats a challenge for the math geniuses
