RH thumb position?

[LarryS]

I've been advised that my thumb position is not ideal, and may cause problems later. I tried moving my thumb to the position in the attached image ( marked thumb----> ) but it felt strange.
I've also been pointed towards getting a thumbport. Anyone use those?

[pied_piper]

Everyone is slightly different. Place a pencil on a table. Without thinking about it, pick up the pencil with your right hand fingertips and thumb. Look at your thumb in relation to your other fingers on the pencil. That's the correct thumb position for you.

My thumb position is roughly between the 1st and second fingers. I am a very advanced player and my thumb is not directly under my 1st finger...