Welcome to the Forum!
If you are planning on using it for hunting,, I have to ask what's wrong with the factory spring? You want to have a dependable spring,, and not a "too light" one where a light strike might cause you to miss a trophy! I own a few Redhawks, & a Super Redhawk,, and I've left all of mine factory stock.
Now,, if you want a smoother action,, have a gunsmith clean up the action a bit.
I bought the spring kit from Midway for around 13 dollars and smoothed up the insides and it is the smoothest revolver I own (haven't resprung the GP100 yet.) I threw in the lightest spring in the kit and it has lit every CCI 300 primer I have used, I do load all my own ammo and do I test it thoroughly. If I have to switch primers then I will test those primers to make sure there is 100% ignition before I use them on game.
You will have to try it and see, different primers take different amounts of force to ignite. Take all the springs to the range with you, start with the lightest spring and if you have a FTF because of a light strike move up one spring and restart your test.
There is just no way to tell if YOUR gun will light off all standard and magnum primers with a 12lb spring. Mine will light CCI 300's which is what I plan on staying with for quite a while.