I personally dont know right off hand as i havent worked with the trx250x very much. But there is a moderator here by the name of trx2five0 a.k.a. moose and he will know exactly where you need to start. You will want to give his as much info as possible. Altitude, ambient air temp, airbox lid on or off, full pipe or just a slip-on? All of the internal mods, piston size, compression, port and polish? Cam? Air filter? As much as possible and he will get you a great starting point. I built one with a friend a while back and we followed his directions and it fired first kick and ran and idled perfect first try. Hes taught me alot about jetting over the years and never steered me wrong. Either wait for his reply or send him a private message. Welcome to the forums!
To many mods to list in signature(ran out of space lol)
Check out garage for full list