What you want to aim for in the ideal world is having the middle ring, of a triple crank, smack inline with the middle of the cassette. On a double ring crank you will be aiming for the mid point between the rings to be inline with the middle of the cassette.
This central point measurement varies across types of bike depending on a few things but someone could help you with a number and then you can do some math to know how much you need to increase the BB.
You can also have a go at putting a meter long ruler/straight edge against a cog in the middle of the cassette and see where it falls in relation to the spider of a fully fitted crank( without chainrings). Remember to make adjustments in the calculation accounting for the ruler/straight edge not actually being the center of the cassette. I hope this makes sense. I am cr@p at explaining things.
Yes; you can go too far out and run into problems with the front mech not being able to move far enough out, or return far enough in. You will also get some extreme chain angles where you don't want them.
My top tip is buy a cheap and brand new Shimano BB like a UN300 to try it out. You may have to buy two because cranks can sit 1-3mm's further onto a taper depending on wear in the crank. When you have the right BB length nailed you could then look for something a bit posher, like titanium, but dont be tempted by the cheap ti BB's on ebay. The tapers and bearings are rubbish.
You can't go at all wrong with Shimano BB's like the 300. Super cheap, strong, and just work. It comes in 122 and 127, or .5 either side of those numbers, but maybe perfection lies in the middle so there may be a trade off, or you need a custom BB.
Don't skimp on a chain is my other top tip and buy it from a proper bike shop on the high street, or online. There are lots of fake chains out there which can vary from ok to rubbish that are sold by alot of sellers on ebay, for example.
