I don't actually believe that it is possible to truly 'sail upwind' on a T with just a jib. I've tried it on at least 3 occasions and the closest you can get is a reach when it's blowing. Most notably we had to do it at the UK Olympic trials in 1983 when we lost our main halyard lock (wind strength 5-6) and on that occasion we single trapped on a reach back to the beach occasionally lifting a hull, but because there was no support from the main via the leech, we destroyed the mast before we hit the beach .

You physically can't double trap ANY cat under jib only and expect to keep your mast - sorry, that's just the way it is.