Headtube Options (frame building)

I need to order a steel head tube for a frame build and am getting into a muddle over the available options so could use some advice...

The forks I currently have are a straight 1 1/8" steerer, but I want compatibility with different ones in the future. I understand that adapter headsets are available that let you run a smaller steerer in a larger headtube, but I don't want to end up in a situation needing an uncommon/exotic/expensive headset.

So, what do I need? Straight 1.5" headtube? 1 1/8 to 1.5" tapered headtube? The ones I've seen online seem to be sized in millimetres than explicitly for the steerers they fit. Would those options be 50mm and 44mm to 50mm respectively?

Thanks

Actually, this one says 44mm and fits any straight or taper steerer from 1 1.8 to 1.5"...

http://www.paragonmachineworks.com/cgi-bin/commerce.cgi?preadd=action&key=HT2007