You can calculate the required torque to shut off against full differential pressure, but there’s no standard for the valve itself. Having a metal seat vs soft seat alone will make a radical difference in break torque.
You really just need the right type of blowdown valve designed for the service. The Velan Powerball is specifically designed for blowdown. Inconel ball and stem with HVOF coating. Rated up to 4500psi and 1,000 degrees F.
Not sure what the full flow rate of the Miller regulator is. At a minimum you can go with a Cv of 18 based on the 1/4” inside pipe diameter at the inlet of the flow regulator. If you can find a relief valve with a Cv higher than 18, you’ll be covered.
Could also consider adding a needle valve...
Top entry is the tough part. I’ve worked on a lot of projects that had a similar requirement, but ultimately the specifications were changed to accept a 3 piece valve instead of top entry.
Entirely depends on the design of the valve itself. There is too much variety from one manufacturer to another to be able to calculate this. The seat type/material and packing style/torque add a lot of friction before you even consider factoring in the process. One extra turn of the packing nuts...
