Bath Surround / Boxing In, and concealed pipework

Figured start a new thread on this as it's a bit removed from "flexible tap connectors" and can stand alone So I'm boxing the bath in. Just doing what I can until I decide what way around the bath is going etc. It's a timber frame all screwed together so easy enough to dismantle. Decided to taper the long bath side so when you stand close your toes will be sort of under and you'll be a couple of inches closer. It'll be tiles atop the 3/4" ply that the bath sits on. Up the side (tiled

So, to continue I need to finish the floor which means laying the wet room corner to falls. At the moment, as above I've left a bfo piece out of the slab NOMINALLY the right size! Roughly this is how the floor falls to the wall drain: So it needs to fall from "0" to "X". The width at "X" (of the bit with the little inverted triangles on) is 410mm. A couple of PDFs, for the life of me I couldn't print decent resolution JPEGs from AutoCAD (sorry)!