Thank you,
A young and curious entrepreneur.
Basically, respectable angels don't want to control your company, so they aren't going to take too large a percentage - 25% or less. They want to invest in you, not own you.
From there, figure out how much you think you're going to need for 12-18 months runway, and you can compute the pre-money valuation you'll need as 3-5x that. Then you convince angels to give you the amount you need at the valuation you want (and they want you to have). If you don't have enough product or enough potential or enough hope to justify the valuation, they can just go to one of the other hundred startups clamoring for their money.
So don't worry about valuations or what others have done. Worry about how you're going to impress the hell out of the angels so they can't wait to invest in you. If you can wow them, you'll get what you need. If you can't, they'll pass you over. Simple as that.
... and for a some interesting reading, check out: http://whoownsfacebook.com
Some very large companies will plop some money down just for the sake of starting a relationship with a company so that they could monitor that company to make an acquisition offer when the time is right. Sometimes the founders want to keep certain investors secret because it may scare off investments or acquisition offers from the competitors of the strategic investor. If you are a founder you should definitely deny board observer rights and financial observation rights to such investors. They can work against you as a de facto poison pill.
Another option is that there are "gatekeeper" investors involved that used to work very high up in some large public tech company. They discover a new company that may be of interest to their former employer. They make a nominal investment in the company and then make a ton of money back by simply making the right introductions and pushing the deal through. While this conflict of interest may be known to many of those involved, they may not want their investments to be openly public since it would shed light on the conflict of interest.
This is just a reference. It could vary greatly based on the background of your team, the maturity of your product, etc.
The company that I had started basically thrived on who you knew in terms of manufacturing and processing in a given area. The larger the manufacturer the larger the paycheck, no up front cost to the company, only back end at time of payment.
Basically I had two business owners approach me with a rolodex of large manufacturers, any of which would net a larger amount of revenue than my entire previous year of pavement pounding that I did on my own. I "sold" 50% of my company to these two in hopes that we would all have the same passion and drive that I had (I was a college student so money was a big deal.)
As you have already assumed, they wanted nothing to do with the operations or effort involved, or even introductions to these manufacturers, only profits which came from my efforts.
Long story short, 49% should be the limit no matter what since I was unable to rid my company of the leeches and ended up selling my share to them, just to get out.