250 or 400 watts?

[clintyiu]

Hello,

I will ordering a blueline ballast for my new DIY canopy but I cannot decide on the 250 or 400 watt.

It's for a new 46 gallon tank (18"x18"x40") with SPS (hopefully). The canopy will house 2-36 watt PC actinics and the MH bulb will be mounted on a track with a gizmo that will move it back and forth along the tank.

Does a 250W MH (Iwasaki or blueline) throw enough light for this tank?

TIA
Clint