Just made this up but up to you.
(ie: 13s) +0 = -20% of npc
(ie: 15s)+1 = npc
(ie: 16.5s)+2 = 10% of npc
(ie: 18s)+3 = 20% of npc (up to here it's pretty safe to get)
(ie: 21s)+4 = 20% of +3
(ie: 25s)+5 = 20% of +4
...
(ie: 43s)+8 = 20% of +7
(ie: 51s)+9 = 20% of +8
Looks alright to me
