IIRC tester's rule of thumb was to double your 25mile time and then add 5 minutes.
Obviously this doesn't give you any info on power but it worked out almost perfectly for me. I think there was a thread on TTF about this recently.
***Not sure if this is the thread I read before but some suggestions in here: timetriallingforum.co.uk/?showtopic=68527
Also more here: timetriallingforum.co.uk/?showtopic=68527 Is the Lynne mentioned in the second link Lynne Taylor/Biddulph? If so then I'm blown away by her values of 98% for a 50 and 90% for a 100!!