Need a formula to equate values of two columns

Subt_Col_EX_Forum.zipHi!

There are two columns with different values.

Column A negative values; Column B possitive.

The aim: to plus the values from column B with values from column A as mutch as it's possible, observing two constraints:

1) Column A values cannot go above 0;

2) Column B values cannot go under 0

See the file attached.

Thanks in advance!