UCC28750: -300V 200mA power supply on Webench

Tool/software:

I am trying to design a -300V 250mA supply (off line, with isolation). The WEBENCH tool only allows for up to -80V, and it allows for up to 1000V positive supplies. What is the reason for this discrepancy?

I felt that, for a flyback converter,  changing the  polarity of input, and the output diode would give a equivalent negative supply. Or, can I design it as 2 rails 1, positive and other negative. with feedback being taken from the positive supply? 

Thanks,

Sasi