"This is as polished as I can make it," is my guideline. Basically I'm ready for betas when I know that I've gotten as far as I can go without a second (and more) set of eyes telling me the faults I can't see (or the faults I think I see but am hoping aren't that big a problem). That's why we get them. Sometimes some really big changes require a new set of eyes when I'm done again, but I've only needed betas after a post-beta revision once, I think.So when do you guys know that you're ready for a beta read? I've thought "this is as polished as I can possibly make it" several times and sent my WIP out to betas, only to have someone's feedback spur a complete rewrite of a chapter, or the addition of a completely new one. Then at that point, I need to do the beta thing all over again to be sure I haven't ruined the flow of the plot, or created character inconsistencies, or made introduced a new character that now needs more "screen" time to seem believable.
Also, I'm so much happier with my first chapter now that I've posted in SYW and rewritten it based on the feedback it got, but I'm at a bit of a loss as to how to improve my other chapters. It was so helpful to see how multiple critters picked up on the same issues, but I can't very well post every chapter in my WIP and expect that sort of help, and I'm having a hard time applying the concepts of what was said to other portions of my story. Or is that what multiple beta readers are for?
Multiple betas will pick up on big problems, but most likely will not make the same types of line-by-line crits that you get in SYW. Enough practice will get you to figure out the more line-by-line stuff if that's what you're looking for in SYW and betas will pick up on bigger stuff (and more practice will also help you there).