Supine Shoulder Flexion and Limited Hip Mobility

Currently my hip mobility will only allow proper PPT/lumbar flattening with my knees bent to just greater than 90 degrees.  Up until today I've been creating the PPT, flexing the shoulders, then extending my knees until I feel the back coming up (this isn't very far).  

 

Today though I tried keeping the knees locked and allowing my hips to flex as much as necessary (straight legs raising up off of the floor) to maintain the PPT.  This was definitely more difficult than the knees bent approach, but is it preferable? 

coach said you may master the PE with your knees bent but to keep in mind not to move forward until you can successfully lay flat for time

as for holding it suspended, you could do that but i figure either your quads will fail or your abs will. not exactly the point of the PE, so you can gently extend your knees until your feet brush the floor with your quads as close to the ground as possible, focusing on PPT

edit: as in, go as low as you can with your feet hovering and then, bend at the knees so your feet touch the ground. want to make the hip flexors relax into the position, not be tense