I went in search of my previous answers to these questions and just decided it was as easy to answer directly.
Tough meats are tough because they contain a lot of collagen. They're heavily worked muscles so they have a lot of flavour but the offset to this is the time required to breakdown the connective tissue, etc. Knowing where on the beast your meat comes from will help select appropriate times for that collagen conversion.
The speed of conversion is impacted by temperature. It happens slowly at lower temperatures but more rapidly at high ones. If you think about traditional braising versus pressure cooking.
So... To cook your bottom round steak will take longer than would a ribeye as it's a higher work muscle group. Experience will help you guesstimate times more accurately, but I'd start with 6 hours at 130F (assuming that you want medium rare.) At the end of this time, lift your steak from the water and give it a pinch between your fingers. Take note of the feel and watch the reaction of the meat. You're feeling for softness that will denote tenderness. If you're happy with it you can move on to finishing. If you're not, drop it back into the bath for another couple of hours and test it again.
It's a really good idea to keep a journal of your experimenting because memory isn't always reliable. Keep track of cuts, thickness, temperature, time and a brief description of the results and your opinion of the outcome.
And welcome aboard. Hopefully the Magnificent Merlin won't get too fat while you're learning.