Can't argue with your arithmetic, but the usual choice is 650VA. In fact, I ran mine with a 500VA toroidal for some time, until it failed. I don't believe that the failure was due to overloading - just one of those things - as it never became particularly warm. I went for one size up because it cost very little more and fitted the space available. There are a few factors here which mean that you tend to get more volts than you would expect. One of these factors is that the transformers will often deliver 5% or so over the nominal value on the basis that it will drop to nominal at full load. 2X35V in series will give you more than you expect, and input mains voltage is also often above nominal so the output volts go up a bit there. Keep this in mind when you are looking at the max voltage input of your drivers.