SubUV Isn't Displaying All 16 Tiles

What should the In Value be labeled as though, since for a 2x2 setup, docs reference a setup of only two points:

point1:
invalue 0
outvalue 0

point2:
invalue 0.75
3.01

how many points should the 4x4 have and should each point be relevantly close to the 0 15 point?