Gym-n-b; I am not sure how to answer as the Cast to HS involves a ratio of the "three" that changes thru the skill.
Imagine you are at the beginning of the cast (feet in front of the Bar, chest down, wrists shifted back (weight on base of palm and heel of hand), lower back round and very tight abs). In this example, a small moment in time, it is nearly impossible to separate all three. The timing of the leg drive (shape change) has to be in sync with the strength and push against the bar, the technique of shape change and shoulder location during the dynamic movement is constantly in play.
The tools to affect change: As a coach, I can attack the strength during conditioning, side stations during other events and then at UB's with drills or spotting thru whole skill. I can work the timing via drills and spotting. I can work the technique with different drills/stations or by spotting. Since execution of the skill, when learning, changes frequently I like to master a few drills/stations/spotting stations so I can better measure consistency and the efficacy of my coaching and athlete learning. I am not being purposely obtuse, by not listing specific drills/stations, as their are many and situations need different set-ups/emphasise.
Pointers towards success: As many other coaches have posted on other threads, being able to preform a straddle press HS repeat (more than 1 in a row) is a good indicator of strength, technique and to a smaller degree timing. Conditioning with excellent technique should be every athletes/coaches goal. Now to this conditioning exercise -it can get more complicated - what if strength is not enough to perform a small part of the press HS? Break-down even smaller. Check with your coach as they have other breakdowns that can focus on strength/technique for that small part of the press. Baby steps... life as an everyday coach/gymnast is an exercise in patience, effort and optimism.
How to help as a parent: Check with DD's coach.
Best, SBG -