I actually had to use a breaker bar on mine, but that was because it was really hard to manipulate. (I believe it was a 3/4 bar, that sound right Lucky? Not sure how that would translate to mm)
It also helps to remove or peel back the passenger side wheel well plastics so that you can properly and easily mount the belt on the lower pulleys that can't be easily seen/reached from above the car.
Bottom line is, if the belt is not tight after you have mounted it and let go of the belt tensioner, then it isn't on right.
Make sure that you have the belt on correctly because if you bypass certain pulleys, or mount in on the wrong side of the pulley, then you can have engine failure.
Here's how the belt should be routed...