I learned this trick from an oldtimer many moons ago, and it works very good.
Attach a length of plastic hose to your battery syringe. (I prefer to adapt it down to a small size, no larger than the syringe)
Slide the hose down 1 side of the fill inlet till it hits the ledge, which indicates the max fill level.
Then proceed to fill the battery using small short strokes of the syringe.
Whenever you get air back on the vacuum stroke, the battery needs more.
But as soon as you get a slurping sound, you will know that the level is flush w/ the end of the hose, then go to the next.
If you prefer a lower level, you could move the hose down a touch I suppose.
And if you are the type that is afraid that transferring a touch of electrolyte from 1 cell to another is going to kill, this may not work for you.
But in reality, once you get the hang of this, there isn't much that can work better.