I was told about the same as you were, but there are no absolutes. To keep my Murphy from looking absolutely unhealthy (too skinny / ribby), we had to really up his food intake.
Here's roughly how his food was adjusted:
3 months (Very ribby) - 2.4 cups per day (4 feedings)
4 months (still very ribby) - 3.75 cups per day (3 feedings)
5 months (still very ribby) - 6.25 cups per day (3 feedings)
6 months (still quite ribby - 8.5 cups per day (3 feedings)
7 months *current* (healthy but still ribby) - 8 cups per day (2 feedings)
Food was increased very slowly the amounts above were at the exact date. I asked the same questions about food intake, and all I can say is they all seem to be different.
Here's a growth chart for Murphy
http://quicklx.stangnet.com/images/Murphy/Murphy%20growth%20chart.bmpHe is now 7 months, 93 lbs, and 27.5 inches tall