measuringslopetile
I am planning to lay a blue stone tiled terrace at the back of my house that needs to run in a slope away from 2 joining walls. The diagram might explain this better.
How can I mark out this sloping gradient so they terrace slopes away from both walls. Also what is a recommended gradient for such a terrace to ensure rain runs off?
Joe, sorry to use the "answer" function again instead of adding another comment, I wanted to attach more pictures.
The tile I used was not available with a bullnose so I finished it flat on the sidewall. The edge cant be seen normally so I did not spend time making it perfect, but for your situation I retract my note about never finishing an edge with caulking. If you end flat to the wall around that corner it would be perfectly fine to use caulking for aesthetic purposes, use paintable caulk so you can paint a straight line down the whole side when you paint the wall.
Sorry I misunderstood your question, my situation was similar to yours and wrapping the tile around to the outside seemed like the best solution for me, and it looks pretty good that way (at least my wife did not complain, that's all that really matters). 
You did hear correctly: the floor must be as flat as possible (and as rigid) in order to support the tile. You mentioned the floor being supported on posts. If there is enough head room a better repair is to shim-up the low spot from below. Depending on how severe the floor depression is this may entail hammering some store bought shims under the joists or installing concrete footings so as to support new posts.
The quick easy solution is to outline the low-spot of floor , mix-up a bag of self leveling floor mix, and trowel it into the depression feathering the edges.
It would also be a smart idea to check the other sections of floor that will get covered by tile for any squeaks, bounce or shifting. Nail or screw to secure them to joists.
When you are ready to begin installing the cement board it would be wise to embed the 3' x 5' x 1/2" cement boards in a thin slurry of thinset for complete support. Use cement board screws to affix them to the floor.
Best Answer
A quick check says a slope of 1/4" to 5/8" per foot.
So over a 8' wall length, that's an elevation drop of 2-5 inches.
But, you need it to slope away from both walls. (Dual pitch) That means that you'll need to go another 2-5" deeper for the free corner. You could get away with the 2" drop along both walls, making it 4" deep at the corner.
OR, You seem to want the water to flow off to the left. I'd go with one course of tiles on the left hand wall, carefully graded at the maximum slope (5/8" / ft) to keep water away from that wall, and then do a single pitch away from the other wall. (Say 3/8"). If you lay it carefully, you won't notice the difference in pitches.