A few thoughts:

One-size-fits-all won't work. But I don't think you're asking for that. The level of incidental take will likely vary among fisheries, species, gear type, and location. It could be fairly low for some species (1%) but much higher for others (10%). Not sure you could ever settle on something simple, like 5%.

But your second question is the bigger challenge. How does one get this into management? Implementing an incidental take restriction is fraught with peril. Look no farther than recreational fishing. When I'm fishing for spring Chinook, I also catch wild steelhead, which I am required to release. How does anyone know if the wild steelhead that I release survives to spawn? The fact is, nobody will ever know. So it has to be an estimate. That estimate will likely have confidence intervals (error bounds) that are huge. So, will we open and close fisheries based on estimates, with large margins of error, and no actual data? Yikes!

If not, then who has the $$'s to monitor the incidental take with enough accuracy to determine when the limit has been reached? Remember, WDFW operates on license fees and taxpayer dollars. Both of which are scarce, despite Fish Doc's recent rant on Washington State taxes.....