so, I found out that w.c.h stands for west coast hemlock - so I have hemlock and not cedar.
I measured the floor flatness with a straight edge around different parts of the kitchen. for the most part it's very flat (0). However, there is one floor joist which is making the floor 1/4" out of plane in 10'. so pretty flat floor except for where this one joist is located... could I use 18"x18" tile or is that still considered "large".
My span chart does not show west coast hem. So I selected Western Woods grade 3 and got a max span of 8'6" to meet L360. That is not good in your case.
I then selected Hem-Fir, not much better, 9'10".
I then selected Hem-Fir North, again not good enough for you, 10'7".
And remember this is grade #3 which should give better results than the grades you have. If the two Hem- Fir were grade #2, you'd be a little above specs.
So, how does the floor feel? Can you sister the joists, can you shorten the span?
As for flatness. Just fix that one area and I guess the floor will be flat enough. Remember, within 1/4" in 10' and 1/16" in 12" of plane. This is for smaller tiles. Large tiles should be within 1/8" in 10". 18" is large.
TILE GUY - retired- TROY, MI - Method & Product suitability consulting.