I'm very curious how you make out on this. I planned on doing what you're doing, except I took things much too far in my enthusiasm. That may even be an understatement.
My only advice, and I'm experienced, but not an expert, is to consider overall, what type of percentage is actually similar content. I personally look at images, image names, css names, image sizes and keywords. You just have to guess what indicates duplicate content.
Based on what I'm dealing with, I wouldn't do this again. I would build one site, then move slowly beyond that. Gaining market share via more domains is a strategy that ultimately will sink your online business. It may be temporary, but it will face destruction if you do cross some sort of invisible line of duplicate content. You have to be totally honestly with yourself about whether you can legitimately provide enough different content. The ratio between sites is a guessing game. If you are only doing a home page differently, then you are taking a huge gamble imo. Somebody once said, just because you're speeding and haven't got a ticket doesn't mean you aren't speeding.
If you push too far, then you may face multiple site penalties. Then you will have the joy of guessing how far you have to reduce your domains/websites/business. You will keep dialing back, removing content and or domains, and then wait and wait from Google to see if you've done enough. Again, please post back after a while with your progress/experience. Thanks.