Using PI Expert enter the input voltage range as 250 VDC to 375 VDC and complete your design. Use this desig to generate your transformer and to select the TOPSwitch.
As far as the MOSFET is concerned select it such that the total voltage across the TOPSwitch and the MOSFET is at least VIN + 2*VOR To be on the safer side I would select a MOSFET rated for 600 V at least. Obviously the MOSFET should have the current carrying capability similar to the TOPSwitch.

Spock,

(I am seeing a Star Trek naming convention with PI's forum moderators!)

There is no change in the transformer design for 250-375 versus 250-700? 

Hi Wes.

Spock retired. Star Trek? I don't know what you're talking about. Must be a coincidence.

With the transformer parameters, we must first be sure we can deliver the power, and the hardest part is at the lowest input voltage. So purely from that point of view, there really are no differences. Because of the higher voltage you may need to have different creepage/clearance values, and that may influence your transformer bobbin (or you may want to use flying leads and be done with it).

You should design your supply so that it is at least a bit continuous, so that you don't get a too low duty cycle at VINMAX, and you should pay attention at your bias voltage to be sure it won't go too high due to leakage. But on a first pass there really is not much more you'll have to look at

It will also influence your choice of output diode, since it will now have to reverse block a much higher voltage (NP/NS * VIN_max) + Vout + tolerance. And of course other components must be rated for the input voltage.