How is my time decided if I touch the wall but miss the touchpad?

Your first legal contact with the end wall decides your time, not when you hit the pad (NFHS Finish Rules Best Practices, Rules 1-1-1 and 6-3-2):

• The wall includes the vertical end wall, the deck and overflow gutter next to it, the front of the starting platform, and the touchpad. It doesn't include the sides of the blocks or the open space of the gutter.
• If you touch the pad first and it works properly, the pad time is official.
• If you touch the wall first (or never hit the pad), the pad time isn't used. Your time comes from the button (semi-automatic) times, or from the watch times if there are no buttons, provided other information such as the order of finish and times in other lanes supports it.

Timers are told to stop their watches and buttons at your first contact with any part of the end wall, whether or not you touch the pad.