If your antibody titration goes so low that you cannot pipet one 'test' (generally, less than 0,5 µl), you can and should dilute your antibody. You can do this simply in your staining buffer (mostly, PBS with BSA and often EDTA).

Storing diluted antibody is not a good idea. It is better to dilute what you need, the moment you're doing the stain, and keep the stock vial in original concentration.

Do not store the antibody solution unless use within 24 hours, but always preferably by diluting at the time of use.

There is no problem in you prepare an antibody solution with a total volume but in some cases only one "tube" requires that label and the preparation should be with specific volume for a tube.

If you want to stock your antibody solution, make the ideal storage condition. Usually the stock buffer containing azide and its dilution should be made in this cap if you want to stock up.

There is no easy answer to "how long can it be stored once diluted". As answered above, it is generally not recommended. I usually make fresh dilution before each staining and use them within a few hours.

But in your case, you might still be able to test it out (how long the dilution is still good, eg. days/weeks) for your future reference in case you use this antibody at a tiny volume very often. 

The above answers cover everything really, just want to add that I am using only PBS (no BSA or any other additives) and worked fine for me, and as mentioned immediate use is better.

You can do both ways.

Add the antibody directly to your sample already resuspeded in a FACs buffer or

Pre dilute the conjugated antibody in a FACS buffer solution ( PBS + 5 to 10% FBS) or PBS+ 1%BSA. You should take a look at the data sheet recommendations for each antibody. I strongly recommed to do not  store diluted antibody. Run immediately after ditution.