Advice please for 'wetroom' tiles with inadequate fall for water.

[macten]

Could people please advice how much slope would be required from the back wall to the drain, a distance of about 156 cm, as I have drawn in the diagram below, for a good drain of water?

The minimum recommended fall is 15mm and the maximum recommended ratio is 85:1, (i.e. for every 85mm the incline travels towards the waste outlet the floor level will fall 1mm.)