Setting an acrylic tub

I just picked up a Lasco soaker tub for an alcove istalation. The instructions call for a bed of builders plaster or plaster of paris to be spread under the tub. Most things I ead mention using thin-set mortor for the bed. Should I use what they say, or go with thin-set? Also, how thick do you lay the bed, as the tub still must rest on the side ledgers I wouldn't want to make it too thick.

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...

You should use this topping/sand mix several of the people here and on JohnBridge.com told me to use it.
Estimate under the tub how far it will be from the floor to the bottom in different spots and put some piles of the topping mix (mixed up with appropriate water of course) that are about twice as high as the space underneath. So if you think it's about 2" from floor to tub make a 4" pile. Then set the tub in and smash it down get a heavy friend to get in there if needed. If you can't smash it all the way down pull it off and remove a little. You have a little time before it dries. I just put in a Lasco shower and that's what I did. I actually wish I would have put more of the mix in for peace of mind I would have been happy to see it oozing out around the edges. Anyway that's how this DIY'er did it.