I really can't help you there. It's difficult to determine how exactly "chance" (as in, the chance of stealing an item) works in the game and how it's affected by save states.

In general, I'd say you would have a greater chance of stealing a certain item (in this case, the Mythril Sword) by stealing as many times as possible in the same battle as opposed to only stealing a little bit and then starting over by reloading a savestate over and over again.

If you just keep stealing in the exact same battle, eventually the game is going to recognize that you've failed in stealing too many times and will give you what you want. The only trick then is being able to stay alive long enough in the same battle.