Above Winkwell and there is more water than you would ever need. It is to keep the bottom pound topped up.
When boats where transient not a problem. One lockful each way but with start, start, turnaround etc. the water flows are not as designed hence the short pound gets low. So the extra water keeps it topped up.
Anyway LEAVE the GATES Open unless you can see signs of severe leakage and we shall all get along nicely and efficient with no hassle. Have a Poll how many times have you just finished closing a gate when a boat has appeared !!!!
You will also notice that the stupid signs from the 80's when the managemnt of BW was mis-guided are disappearing and being replaced only where it is necessary.