Formula with increasing value ranges - plumbing code calculation

[cellophane]

I'm trying to put together a spreadsheet to do some plumbing calculations for us based on the Kentucky Building Code (which has stupid fixture calculations btw). The code states that for every X increase in occupancy we need to add an additional fixture and for every Y increase over a certain value we need an additional fixture (varies by occupancy classification.)

A. Occupancy = 100 or less - 1 WC
B. Occupancy = 200 or less - +1 WC (2 total)
C. Occupancy = 400 or less - +1 WC (3 total)
D. Occupancy > 400 - +1 WC for each additional 400 people

I understand the formula in theory but have no clue how to actually write it...

if A<100 then 1, if A < 200 but > 101 then 2, if A > 200 but < 400 then 3, if A >500 then 3+1 for each additional increment of 500.