Hi and welcome to the forum.
The rule of thumb for woody plants, that is, shrubs and trees, is 8 inches of rootball per inch of trunk diameter, measured about halfway to your knee...crepe myrtles are very tolerant of being transplanted, so you've got a very good chance of keeping it alive. Another rule of thumb, dig the new hole twice as wide as the rootball. I would throw in some bonemeal, but you don'tneed to, and it's not good to amend the soil with peat or topsoil, the plant will adapt to it's new location much better if you don't.
By "main root,", I'm thinking you mean a taproot? Don't even worry about it, crepe myrtles don't really have a "main" or tap root, just get the new hole prepared, dig the crepe myrtle and get it transplanted quickly, and I can almost guarantee it will be fine. The root system should be dug roughly half as deep as the width of the "ball," deeper is good, but you'll get most of the major roots going, say, 10 to 12 inches. You are transplanting one of the easiest shrubs of all, just keep adequate moisture on it. :)
