Mine's has about 800 megs of space left, so I'm guessing that if you have around half a gig (y'know, give yourself some wiggle room since you're adding, deleting, rearranging, etc), you should be fine. Get Free Memory. Clears up your RAM.
I don't think it matters how much free memory you keep on there, I currently have a little over 100mb left on my 32gig, and everything runs as smooth as butter. What matters is your RAM, I think, which is the cache of memory set aside just to start up and run apps, etc. That's why rebooting your device after downloads is a good idea, it frees up that memory to run the game you just got. I also use MemoryInfo which you can use to manually free up memory without a restart. It says it only clears up to 35mb, but usually it'll clear around 45-50mb for me. It's a pretty handy tool. But as far as the capacity of you device, you should be able to fill it up as much as you can without causing any problems. I'm no expert or anything though
Since it is a flash based NAND chip, it has a flat seek time, so 100% free, or not a drop free, everything runs the same. You'll want some extra for your own convenience (TTR songs, new apps, misc. data), but otherwise, space remaining doesn't affect performance at all. It's also a thing with OS X that helps that. OS X has a more natural, all together setup, which iPhone is based from. All that keeps things running like the day you got it!
