for negative numbers you first have to make sure the midi value is negative.
For example Roland's SonicCell has Master Key Shift with a range of -24 to +24 but they use a Midi data range of 40 to 88.
Roland take Midi data 64 as being the parameter = 0
60 - 24 = 40 and 64 + 24 = 88
But if it is really negative Midi values perhaps this is a good place to start.
All the best
Royce