Hi !
I don't use TransX, i use Interplanetary MFD (IMFD). A good advice for a direct mars mission is : make a correction as soon as you're out of Earth's SOI. Then, when you do the plane correction (if needed). After i use a trick : i look regularly at the AutoBurn (AB) screen. The burn duration could grow during the flight. When it becomes stable, i do the burn. It's often enough to be OK until i start the Planet Approach program. But sometimes, 3 or 4 corrections are needed. Sometimes, the post SOI burn is enough. As N_Molson said, a lot of things interfere in your trajectory : gravitational influences, vessels' thrust, MFD's computings, pilot's patience... In Orbiter, you can't simply do the burn and let it go until arrival. You have to keep an eye on the numbers during the flight. There are good tutos on YT about IMFD, in english. Don't forget to save your game after very step. Some addons provide multiple scenarios for one mission, so you can train yourself step by step, from launch to arrival / landing. I hope those advices could help you.