Weight isn't going to be very easy if you have different bottles. You'll need to weigh each one, then add 757.5 g of a 1.010 beer. 765 mls if the beer is @ 1.020. [required volume * density (specific gravity)]
If you really need consistency, then I'd recommend standardizing your bottles first. Find out how similar the bottles actually are: how close in weight, or how far up the neck does a measured 750 ml actually come?
Then you need to figure out what an acceptable margin of error is. + or - 1 ml? 2 ml? 10 ml?
Given discrepancies in your bottles (whatever that turns out to be) how much liquid will you need to put in them to ensure that they have at least that much in them? For example (pulling numbers out of my ass here), if a mark 2 cm down on the neck has a 5 ml difference between bottles, you'll need to add at least 755 ml to make sure all of them get 750 ml (if that is your goal). Or not, if an acceptable margin of error is 10 ml.
Then devise the best way to get that for the size of operation you are running. You will need to figure out what the error is in this method, too. If it is say +/- 1%, and you again need to get AT LEAST 750 ml beer per bottle, you will again need to overfill by a little (~7.5 ml) to make sure that each bottle gets the minimum amount.
Or something like that. My head is starting to hurt... It really depends on what your goals are. IIRC, you are looking to go pro? Which makes me think you might need a bottling plant. Seems to me microbreweries start out with just kegs for this reason!
