Excellent question, as most studies have found 7 out of 10 car seats are incorrectly installed.
Most areas/ states/ otherwise do provide safety seat inspections. People just have to look up where and when.
I think you mean the shoulder strap retainer clip, and it should be at armpit level, basically at nipple level. Checking the tightness is also important, you should be able to barely slide a finger under it. You don't want it so tight it leaves a mark, but not so loose the child can slip out. In cold weather, strap the baby snuggly in, then add blankets on top.
And in your case, even if you weigh little, you should be able to hook your seat in tight enough. Do you get in the seat and put your weight down with your knee??
Scary what you see out there. At one seat inspection, we had a grandma come in with her 3 yr old in an infant seat. And booster seats are way under used.