ADB cannot be negative lah. ADB change over previous month can be negative. It is very straightforward, there is no rocket science.
Look at my example below.
1st Apr balance was 1k
2nd Apr balance was 1k
3rd Apr balance was 0.
So ADB as of 3rd Apr is (1k+1k+0)/3 = 666.66
Mar ADB was 1k.
So change in ADB = -333.34
So simple.
If you want to calculate yours just do the same thing, take the end of day balance of each day in a month and divide by number of days in the month. If you opened you account in the middle of a month, then just consider the days from the date of opening. There is nothing more to it.