It's in
C:\Program Files\Microsoft Games\Freelancer\DATA\MISSIONS
Then look for ptough.ini

It should look like this once opened, I have modded this already so you can get an idea of what you can do.

[PlayerToughnessScale
ptough_graph_pt = 0, 0
ptough_graph_pt = 11000, 1
ptough_graph_pt = 137500, 2
ptough_graph_pt = 272280, 3
ptough_graph_pt = 308450, 4
ptough_graph_pt = 453960, 5
ptough_graph_pt = 604750, 6
ptough_graph_pt = 765760, 7
ptough_graph_pt = 838910, 8
ptough_graph_pt = 921830, 9
ptough_graph_pt = 1008967, 10
ptough_graph_pt = 1166632, 11
ptough_graph_pt = 1274156, 12
ptough_graph_pt = 2288245, 13
ptough_graph_pt = 3405206, 14
ptough_graph_pt = 7514780, 15
ptough_graph_pt = 17025225, 16
ptough_graph_pt = 27366200, 17
ptough_graph_pt = 37500000, 18
ptough_graph_pt = 46000000, 19
ptough_graph_pt = 48500000, 20
ptough_graph_pt = 52500000, 21
ptough_graph_pt = 57500000, 22
ptough_graph_pt = 62500000, 23
ptough_graph_pt = 64500000, 24
ptough_graph_pt = 70000000, 25
ptough_graph_pt = 75500000, 26
ptough_graph_pt = 80000000, 27
ptough_graph_pt = 84500000, 28
ptough_graph_pt = 90000000, 29
ptough_graph_pt = 95500000, 30
ptough_graph_pt = 102000000, 31
ptough_graph_pt = 109500000, 32
ptough_graph_pt = 116000000, 33
ptough_graph_pt = 130500000, 34
ptough_graph_pt = 150000000, 35
ptough_graph_pt = 200000000, 36
ptough_graph_pt = 300000000, 37
ptough_graph_pt = 500000000, 38
ptough_graph_pt = 750000000, 39
ptough_graph_pt = 999000000, 40


"It doesn’t matter what universe you’re from. That’s got to hurt!"