"stanley johnson" wrote:
I need to indicate that a sum has a net value calculated as -%. eg =SUM(P20-%)

I am not entirely sure what you mean. I think you want:

=P20*(1-5%)

to compute "P20 less 5%". If that is not what you meant, please clarify.

Obviously you can replace "P20" with any expression. For example, if you
really meant a "sum", you could write:

=sum(P1:P20)*(1-5%)