Please- do not tighten the rear cinch like that. Western saddles have a rear cinch not to hold the saddle down tight, but to keep the back of the saddle from popping up and slapping down. It should show daylight between the cinch and the belly, but not enough that a horse could get a hoof between the cinch and the belly. A good guide would be that you can slide your whole hand between the cinch and the belly.
If that rear cinch slides back, it will hang up on her flanks and can act as a bucking strap.
Most have a keeper strap that connects the front and rear cinches together.