A normal hand clamp handle turn will do, applying too much pressure may "starve" the joint (if too much glue get squeezed out) and too little of it left between the miter cuts to create a strong bond., Pre-gluing porous wood prevents this quickly and easily.
MasterClamp recommend joining method as follows:
make sure no joining gaps (adjust the clamp if need)
apply glue to both miter cuts and align
apply clamping pressure clamp handle with normal force (don't go crazy)
leave clamp 20 for bonding (clean frame from glue)
may remove clamps (and secure w fasteners at any point thereafter).