Do we have a sense for what proportion of text is actually retrievable from these scrolls?

That varies greatly on the state of preservation of the scroll. For some of the scrolls we can recover entire columns of text. But this is a best case. Plenty of scrolls, or portions of scrolls, are extremely damaged and warped to where our current methods cannot unroll them through any combination of automated and human driven unrolling. Both of these still have massive headroom for improvement, but achieving that headroom is hard as the preservation gets worse.

To give numbers, for ideal portions of scrolls, we can read 100% of the characters. In nonideal portions of scrolls, we can read 0% of the characters. It's not really possible to quantify how much we could theoretically recover of that 0% through better methods, and how much is truly destroyed.