Lets talk about what "chainline" means:

1. Ideally, you'd like for the chain to be parallel to the bike's main frame. With multiple sprockets on both the front and back, that obviously isn't going to be the case all of the time. Not to worry because it doesn't have to be perfect.

2. The biggest single factor in determining what BB length you need to achieve that perfect chainline is the crankarm design. Older cranksets tended to need bottom bracket spindles that are quite wide by modern standards. Frankly, I think that 122 mm is pretty wide. Unless there is something else that's unusual in the design of your bike, I'd rely on the spindle length recommended by the crank seller.

3. The second factor in determining BB spindle length is (surprise) rear dropout spacing. Road bikes typically have 130 mm dropouts. Mountain bikes generally have 135 mm dropouts. That means that the cassette on a mountain bike is going to be spaced 2.5 mm farther outward. That means you need to move your chainrings outboard to achieve that perfect theoretical chainline. If your crankset seller recommends a range like 110/113mm for spindle length, choose the longer for 135 mm dropouts and the shorter for 130 mm dropouts.