You don't want thinset...something like deck mud if you want to go that way. The object is to fully support the bottom of the tub to prevent it from deflecting when you step into it. This keeps the tub from getting stress cracks as it ages and makes it feel much more substantial. Thinset as implied by its name, should only be applied in a thin layer, and wouldn't do what you want. In addition, it would actually bond the thing to th efloor, which is not necessary. When all else fails, follow the manufacturer's instructions. Whatever you use, you need to set the tub into it prior to it seting up. THen, if your water is available, you could add some to smoosh it down to the right level. Plaster of paris sets fairly rapidly, you'd have more time with something like deck mud, but either would work. Note, I'm not a pro...