I'm a total newbie when it comes to material melding and I can't quite figure out that one online BiS calculator thing. Assuming I'm a MCH in full proto-Midan minus one Lore ring, what should I be melding and what should I be replacing with Lore gear on the left side first? Should I be putting double Det Vs on my Torrent Musketoon?
Good left side pieces for Machinist are literally everything but the head piece and belt. Chest and Pants are both very good pieces to target first, then the hands and the feet depending on if you need more accuracy or not. For melding, it's pretty simple.
http://ffxiv.ariyala.com/MCH
So with your Torrent Musketoon. It has 114 Crit and 76 Det. The higher number of the secondary stat is usually completely capped. You cannot add any more onto it. So your options for melding onto the weapon are basically Accuracy, Det, and Skill Speed. You can see the small numbers in the boxes on Ariyala. Those are the stat caps for each particular stat.
Det can go up to 109, and you have 76 on it already. That means you can comfortably fit 39 more points of Determination on it before it caps. Since you can only meld two pieces onto it, and +12 is the highest that materia give you right now, It's very safe and a very good idea to meld two Det 5's onto your weapon so you get +24 det, which is the best gain you can give it.
That all being said, it depends on what your purpose is for this gear. If you want to be raiding, you probably want to drop 5's into more permanent pieces that will not be upgraded for a very long time, or for Best in Slot pieces that are maximum ilevel. Otherwise, throwing the cheaper 4's onto your gear is going to be way more than good enough.