Change values automatically

Assuming we're starting in B2

=IF(A2<5,0,MIN(1800, Formula))

where formula is whatever formula you put in B2 to generate the result initially. If that value is >1800, it'll choose 1800. Does that work for you?

NOTE: This formula can be put into column 2. You don't need a column 3. If you want to have column 2 as is, then change the formula to

=IF(A2<5,0,MIN(1800, B2))

and place it in C2.

ChemistB