Solver problem where 'IF Function' is bad



What I'm looking at here is to determine if the salt level in the water
affects plant production.

What I had was



=(B181*(1-(B$130/100)*(IF($V56B$129,$V56-B$129,0))))



Written in English is

B181 = max yield

B130 = salt threshold slope

B129 = salt threshold

V56 = salt in the water



So yield = maximum yield * salinity impact

Where salinity impact = Slope * Threshold Violation



As the Solver model was trying to solve a global solution and the salinity
impact changes as the level of salt changes in the water the threshold may
or may not be violated.

I need to be able to change the Threshold Violation into a number and not an
'If Formula'. It doesn't matter if salinity impact is less than 1 but the
number should not exceed 1.



Eg 1

Salt in the water = 40

Salt Threshold = 100

Threshold Violation = 0



Eg 2

Salt in the water = 120

Salt Threshold = 100

Threshold violation = 20



Any tips would be greatly appreciated on how I can avoid using an If or
other non-smoth function would be greatley appreciated. AS this formula is
currently used in a table of 18*11.