Delta Sigma ADC in ADC Pi (MCP3424), how it converts Analog to Digital
Everything you need is in the datasheet for that chip in table 4-2 and 4-4.  There is no 17 bit mode, but a twos-complement 18 bit mode.  If your gains are set to 1, then 

Vref- 1LSB will produce 0b01 1111 1111 1111 1111

-Vref will sample as 0b10 0000 0000 0000 0000

and zero will be all zeros

You must divide Vref by any gain setting other than one you place on the PGA.

Having an 18 bit word come in via I2C means that you'll either be throwing out some LSB's, of reading the data into a 32 bit word.  Pay very close attention to pages 23 and 24 so you understand what your data will look like.  Since these are twos complement numbers, I recommend SOMEHOW reading them in such that the data is LEFT ALIGNED with your SIGNED 32-bit word, and then you can shift them right if you prefer them right aligned, and the compiler should not have problems dealing with the twos complement format.   Note that if you get dealing with twos complement format wrong any numbers that end up negative (even due to noise) will not be handled correctly.