Small fills are not only inaccurate, they waste your time, your tank is open to the elements much more when the cap is off, you spill more, and you are constantly driving in/out of service stations. No good comes from constantly going to stations for small fills unless there is a cute girl working the register that you simply must see all the time.
I wouldn't advise any particular gallon amount either because of the large variation in vehicle tank sizes. IMHO about the best one can do is only go to gas up when the vehicle is at or below 1/4 tank (as close to E as you dare to get) then fill the dang thing all the way up and be done with it until the next cycle. It is helpful to try to fill at the same pump at the same station every time, and also use the same method i.e. fill on slow auto til it clicks off, wait a bit, then do two clicks... whatever floats your boat, just do it the same way every time. Might even want to do it at the same time of day for temp reasons.
I can almost guarantee you that the potential errors from calculating mpg from small fills are greater than the potential gains from our garden variety mods. Sub-9 or 10 gallon "fills" = waste of time.
Without mpg instrumentation I think the best you can do is keep a gaslog and make notes of mods/conditions for each entry. Don't get excited about any mpg spikes on one or two fills either. We have to be content with noticing trends.
Gas tank games... will they ever end?
