NAI/WAI On Altitude During Decent

I'm curious to know how you can predict the altitude when anti-ice is to be switched on during decent (B744, FORECAST Page). I'm no weather expert, so I'm stumped with this. I assume you're somehow supposed to pre-calculate/predict the tops of the highest cloud layer, and that would normally be the NAI on altitude to be entered in the FORECAST page. But...1. I *think* METAR reports do not report cloud layers above FL180, especially when there are already 3 layers listed below FL180. I'm not sure if this is correct? If so, the METAR won't always list the upper cloud layer in this instance.2. Even if it does, the layer listed would be the cloud base, rather than the cloud top, which is when anti-ice needs to go on.So before decent, when entering the decent forecast winds, how do you know what to enter in the NAI ON altitude entry? I came across some formulas to calculate the base of the lowest cloud layer, so then is a similar formula to calculate the tops of the highest cloud layer?Or, how else is it supposed to be done? :-hmmmThanks,Subin.