You're thinking about this completely wrong. Free RAM does you no good. If you have 300MB free, that just means there's 300MB you are not using. You should want as little free RAM as possible because that means as much RAM as possible is doing useful work.
min_free_kbytes as low as you can possibly get away with so that you can use your RAM rather than having it sitting around doing nothing.
You must be thinking "I want free RAM so I can use it later". But now is later than it was before. And if you use it now, you can also use it later. All that happens if you don't use it now is that you forever lose the opportunity to use it now. Later never comes -- it's always now. Use the RAM now. Don't save it for later because then later you'll want to save it for later too.
It's not like if you use 4GB now you can use 12GB later. If you use 7GB now, all that happens it that 1GB went to waste. You want as little free RAM as possible so your system can get the most benefit from the RAM you have. You can't save RAM. The opportunity to use that RAM now must be used now or it's forever lost.
Limiting dirty pages does make sense because dirty pages cannot be used for another purpose until the contents are written to disk. This can cause the system to be forced to use RAM ineffectively or to do very large bursts of I/O. However, I don't think you really need to adjust the defaults. They're pretty smart.