sorry to bother you all again, but this will be the final time as ive just about finished my project.
my file is here:

im interested in the future part(which works ok)
if you input a future date, but a date no more than 45 days into the future you will get an auroral oval that represents prediction data retrieved from an ftp site

my globe is azimuthal equidistant, it goes down 40 degrees (south england) and it is 285.7 pixels wide and high

my problem is that i dont know what to set my auroral oval radius's to, so that they are accurate

could you offer any advice on how i can work it out?