The correct slot is the one that is closest to your child without being underneath your child.
To Adjust Crotch Buckle Strap (Also located in your owner's manual):
Recline the car seat and locate the metal retainer under the car seat shell.
Rotate the metal retainer and insert it up through the shell and seat pad. From the top of the car seat, pull the buckle out.
Insert metal retainer in the new buckle strap slot. Reach underneath the car seat and pull the metal retainer through.
Pull up on the buckle strap to make sure it is secured.