Hey mate, I did the same thing. The genuine bar is a straight cut from one side to the other, unlike ARB bars and the like that go straight for a bit then go down. I found a car with the same bar and measured it. Measuring inside the wheel arch, I cut mine 120mm down the bumper from the join with the front guard. There's a bit of give in that, the cut can be anywhere from 100-125mm really.
People say you don't need to take the bumper off, and they're probably right, but I did just to be safe. I actually dummy fitted the bullbar without the bumper and double checked my 120mm measurement. Measured the 120mm in at each end then ran masking tape from mark to mark and cut it off with a jig saw. There is some webbing in the back that was longer than my jigsaw blade, so having the bar off was good to get through those bits.
All in all, it was a piece of piss and took me about 2.5hrs start to finish. If you take the bumper off, you will need to remove both inner guards and will also need to trim them at the front when refitting, I just cut them with a stanley knife.
Hope this helps!
Brett
