How do I build a wet room that:
It is very hard to get the slope right by hand and I have need two many cases when the sleep is too steep and the water run runs into the rest of the house.
A Barrier-Free Water Dam might be an option. They allow the user to simply roll over them, then they spring back to their original shape and contain the water to the shower.
While I haven't build a space like you're describing, I have helped users of them and seen their designs.
If you are just trying to build a space that would allow the wheelchair user to shower, there are a number of products that are designed to make this relatively easy to install. A quick search yielded the link above, but I'm sure there are other options out there. These are less elegant looking than most true wet rooms, but can be highly functional. During college I assisted a user of a shower like that, who thought they were relatively well designed except that the controls were not in a location a seated person with restricted arm movement could easily reach (something to keep in mind however you build it).
If you're building a larger space you'll need to grade most of the floor, particularly the area around the shower to avoid water pooling away from the drains or flowing toward the door. This article has some thoughts to keep in mind when designing the space. You can make a raised edge near the shower that a wheelchair can get over just make sure you don't make it too hard to get over. I've also seen spaces designed with a ridge that runs across the room that didn't cause problems for wheelchair users, but caused water to run toward one of two drains in the floor. The advantage of grading the entire floor as I understand it, is that you always control where the water is going, instead of hoping it stays put. This can be problematic of people with balance issues if you're not careful.
Rather than a slope, consider a little bit of a lip. You'll need to consider who it is in a wheelchair, but if they can make it up a small bump, then add a higher threshold, and trim the door to give the necessary clearance.
You might also consider a bit of weatherstripping or similar, so that water can't easily get through the gap under the door. (it doesn't need to seal that well, it can just be a small plastic flap on the inside of the door that comes down to meet the edge of the threshold.
