That might not even be enough to eliminate sweating. I suppose if you know the maximum dew point the room will experience, and you know the surface temperature of the 1" thick plex at that time, you could predict if the tank would sweat or not - but in reality, what most people do is set the system up and see if it sweats. I've seen air blowers used to reduce minor sweating in some tanks - just blow air across the front of the tank.
Do you really need to keep the tank in the upper 40's? Most animals I've worked with can go a bit higher (say perhaps mid-50s) and this reduces sweating quite a bit - more than you would expect for just a few degrees rise. Giant Pacific octopus and truly Arctic critters need to be kept very cold, but virtually everything else I can think of will do just fine slightly warmer than that. I never advocate "pushing" the temp limits on animals, but keeping them at the high end of normal should be no problem.
We use a frame and a sheet of glass or plex over the entire tank front - spaced about 1" from the front panel of the tank. Then, we pump dessicated air slowly into that space. This reduces the dew point, and no matter how cold we go, there is never any sweating. The problem is getting enough dessicated air - we use a regenerative dessicator - probably costs $3k now. I spent $600 this summer on a re-build kit for it (has to be done every two years or so).
One *possible* solution would be to create the same air space, but seal it, and include some dessicant crystals. This will dry out the trapped air, keeping it from fogging - that is until the seal leaks room air into the space and you have to replenish the dessicant.
Jay Hemdal