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After recently purchasing a Mack Brass 410, I want to modify the thumb ring so I can properly use the 5th valve without having to move my whole hand. Where should I start with this?

You could have an adjustable thumb ring (linked below) installed -

http://shop.voigt-brass.de/en/COMPONENT ... -ball.html" target="_blank

It adjusts up and down. I would either like a larger circumference or a larger D shaped one. I do have access to a machinist, but worried it might turn out bad.

I will get a picture tomorrow evening.

The problem is I have a short thumb. I would like to move it closer to the valve or the valve closer to my thumb. I prefer moving my thumb closer to the valve, so my hand would be in a more natural position. After turning the thumb ring a little last night, it does feel better.

Custom tailoring. Everybody's hand is a different size. Everybody's fingers have different lengths. When I got my 186, I spent an hour or so with my tech so we could adjust the four valve paddles to fit my hand so I could relax and play and not worry about fiddling, stretching, etc. The OP's thumb is no different. The solution is to spend time with a good tech who understands ergonomics. Remove the ring, adjust the four paddles to fit the hand, then analyze the position of the thumb, the position of the trigger to see if it needs adjusting, and then re-solder the ring as appropriate to fit.

Thank you for the info. I have 2 techs fairly close, so I will be getting in touch with them.