newTargetTime=targetTime-((targetTime-actualTime)/nominalDifficultyValue)
so
targetTime=2 minutes / 120
actualTime=1 minute 30 seconds / 90
newTargetTime=120-((120-90)/difficulty)=117
a difficulty of 10 would be
newTargetTime=120-((120-90)/10)=119.7
So being 30 seconds quicker would reduce the target time by 5 seconds.
Being 60 seconds quicker would reduce the target time by....
targetTime=120-((120-60)/10)=114
You probably want a value lower than 10 for your nominal difficulty.
so
targetTime=2 minutes / 120
actualTime=1 minute 30 seconds / 90
newTargetTime=120-((120-90)/difficulty)=117
a difficulty of 10 would be
newTargetTime=120-((120-90)/10)=119.7
So being 30 seconds quicker would reduce the target time by 5 seconds.
Being 60 seconds quicker would reduce the target time by....
targetTime=120-((120-60)/10)=114
You probably want a value lower than 10 for your nominal difficulty.