@Ring; Its looks hard to decide but I think your fault is little more than XRG driver. You have clear left, and same goes for XRG; he had almost clear left (you had no overlap), than at the braking zone he brakes little early than you as all XRG's do normally, you should known that & should have been prepared for that also.
I said hard to decide because the moment & position of the XRG when just starting brake (nose pointed left-according to the track longitudinal axis) is not what ppl generally do. This point is debatable but still since you hit the XRG, I can say that its mainly your fault because its braking zone.
And for the ppl who thinks its "big no no" to braking front of someone, you need to reconsider and think about the yellow flags, some abnormal conditions, IRL, etc. And its normal for ppl having different braking distances, so, be prepared for early braking at braking zones.
IMO, best thing is don't wait for the front car's stop lights, brake just little earlier and adjust the brake pressure by keeping the safe distance.