Statistically I don't know what this is called, but what I'm looking for is a
function that will hold a certain fixed realtivity to zero among the
remaining set of numbers if one number is changed. For instance:
1,.-0.5, -0.3, -0.2. these combined net to zero.

if I change 1 to 0.5 I need the other numbers to hold the same relative
distance from zero and net to zero (if that's possible?).