When installing a forward facing car seat with shoulder belt, is it more important to get the belts super tight or have them straight?

I installed a britax marathon g3 in our new nissan pathfinder in the 2nd row, forward facing. I'm using the shoulder seatbelt. I pressed the car seat against the regular seat with my knee as hard as possible and pulled on the shoulder belt as hard as possible to remove any slack at all. Well of course pulling this hard the seatbelt bunches around the frame of the car seat. I still managed to get the lock off down and the car seat doesn't wiggle at all, let alone an entire inch they say is the maximum allowance. So my question is, is it ok that the belts are kind of bunchy around the tight areas? Or should I loosen it for a straighter belt? You literally cannot get it tight so it doesn't wiggle at all without having it bunch slightly around the frame cus the seat sits higher than the belt and the tightness pulls the belt down around the frame parts. I think tighter is better but wasn't sure.