changing a value

how would i convert a value in ladder logic
say a = 2000 i want b = 1
then say a = 4500 i want b = 2
then a = 8000 i want b = 3

what i am doing is using a count up to turn on and off valves at different times and i want to add a hmi step switch with a another counter. so step 1 is = 2000 on the first counter, step 2 = 4500, step 3 = 8000. also keep the plc running from the last step enterd. i have it working with the counter but i have to push the step button a lot to get from one step to the next. i just cant add values cause the valves trigger at different count times.

A table or scale function? Linear?

how would i convert a value in ladder logic
say a = 2000 i want b = 1
then say a = 4500 i want b = 2
then a = 8000 i want b = 3

its not liner or scale or table i cant use math.
in c++ i would just say if a = 2000 b then = 1

In current PLCs there are compare blocks for use in ladder.
Something like this: