I've been messing with nitrogensports.eu the past few years but I still don't know what the minus before the number means
so when you say they gave you -105, what does that mean exactly?
That's the juice in which you pay for losing bets and it's also your break even rate.
So you lay 105 to win 100. Theoretically assuming there's equal action on each side, The winning bettor gets 100, the loser gives up 105 and the 5 goes to the bookmaker for booking the action. Unfortunately the books rarely get equal action and either win or take a loss based on how the action is booked.
-110 is the industry standard
At -105 your break even rate is 105/(105 + 100)= 51.2%
At -110 your break even rate is 110/(110 + 100)= 52.38%
At -115 your break even rate is 115/(115+100)= 53.48%
At -120 your break even rate is 120(120+100)=54.54%