The middle value on the home screen is designed to help users stay on track with reaching their daily water intake goals by the end of the day (by default I think the "day start" time is 8 am and the "day end time is "10 pm").
For example, if your daily goal is 80 oz, then around 3 pm you should have reached half of your daily goal, which is 40 oz (under default settings).
If you're at 35 oz, the H2OPal will say "5 oz to stay on track", but if you're at 45, you'll see "+5" and "You're on track".
Here are screenshot examples:
The white line on the radial progress bar shows where you should be right now to stay on track of reaching your daily goal by the end of the day.
In essence, if you're already drinking enough water the middle value is showing you how much more you've drank compared to the right amount of water given your total daily goal and the current time.
Explanation with screenshot (without radial progress bar):
To manually log your intake simply touch and drag the circle on the screen upward until the indicator reaches the amount you'd like to log.
Video walkthrough showing home screen, profile screen, list of logs, etc.:
To add a manual entry simply hold and drag upwards the large circle in the bottom half on the home screen (see video below).
H2OPal during rest time
H2OPal is designed to help you achieve your daily goal throughout the course of the day.
After the day has ended the app is in "rest time".
(At the moment, the latest you can set your rest time to start is 11 pm).
Here's a picture with explanations regarding the "rest time" screen:
Your water intake during rest time will still get counted and it will still show in the "list of logs".