How can I word "add bread crumbs until it feels right"?
I see two things to address, first identify a few physical properties of your ideal meatball mixture, and second have a verifiable way for the reader to compare their current mixture to what the meatballs are "really" supposed to be like at that stage.  

Depending on how many words you want to use for this part of your recipe, outlining a very simple "test" for the reader to try may be helpful. Maybe a sidebar section on the page with some instructions would visually unobtrusive, allowing those users who've already made your recipe to focus on your other recipe steps when needed.  

What comes to mind is using a tablespoon to scoop out a quantity of the meatball mixture, followed by inverting that spoon onto a surface(not slamming, inverting).  The ideal meatball mixture probably behaves in a certain way(doesn't spread out more than "x" centimeters/won't leak moisture/etc...I suspect you can come up with the right variables to emphasize).  

Another example would be to say, "...when the mixture is right, a golf ball sized portion should be easy to roll into a ball and not begin to sag within "x" timeframe(seconds?), or something like that. Pick out the physical properties of the mixture that make the consistency or shape right.  Work backwards, try a few trial run "tests"by making a tiny batch that has too much  or too little water/fat/breadcrumbs/etc.  How does that incorrect-ratio-mixture behave with your tests?  

Off the top of my head, the easiest descriptors to identify would be: moisture content(how easy is it to squeeze out liquid &amp; how much should come out), consistency comparisons(how much sticks to your hand/feels like wet spaghetti/whatever),and height/spread under specific conditions.
Hope this was helpful.