JeanneS's description is how I think of it too. My gran tied all of her quilts and the way she taught me is that if you drew an imaginary circle around each tie, that circle should touch another tie in each direction (top, bottom, left, right). Her batting had a 3" max, so when she was teaching me, she found a six-inch wide plate and that was my "tester" to make sure I was spacing things properly.
So translating that to machine quilting and your 8" quilting allowance batting...unless you can fit an 8" plate somewhere on your quilt with zero quilting touching it, you're totally good to go!
