How do I mark out gradient for tiling?
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.