So the proper foot placement for the ollie is with the ball of your back foot horizontal on the tail of your skateboard. And your front foot centered on your board right behind the front trucks. You may find that moving your feet around in different positions are more comfortable. This is fine, everybody can learn how to skateboard their own way.
To get higher ollies, I would suggesting resting your back foot on the very end of the tail and the front foot in the middle of the deck to allow for more slide. Popping as hard as you can and catching your board in the groove of the nose with you ankle practically resting in the groove will level the board out in the air. after this initial movement slide you feet back to the bolts on the tail and nose to ensure a clean landing.