Could you take your 16-bit measurement value and divide by the number of steps you require? Then use a round() function to get the nearest step. [EDIT: I was tired when I wrote that… I don’t think that’s the correct math.] You can include the math.h library as described here: Round() Function Error - Web IDE [SOLVED]. You can use any of the c++ math.h functions as found here: http://www.cplusplus.com/reference/cmath/.
Actually, using the map() function as described in the docs might get you to where you need to be. There is a discussion on the map() function here: Help 4-20mA Convert Data Code and Adjust Readings