Don't know if this will help because remember I am coming from a meterless manual camera but with that particular camera it has no ISO or ASA as they called it back in the day...what speed of film you put in it is what you get period. So for me ISO/ASA is only relevant (on that particular camera at least) if I was going to put a metering head on the camera. I just usually sunny 16 it with the shutter speed set to film speed and at F16 for a baseline daylight shot. So then you ask well what do I do with my D5000 which I shoot only on M concerning the ISO?..well if there is lots of daylight I shoot it at 160 or 200 ISO, if I'm inside I try to shoot it at 400 or 800, and if it is dark I try to shoot it at 1600. Sort of the same way I chose film speeds...Please remember that 400 ISO is still pretty darn fast even with a modern camera but the sensors are getting much much better at this aspect. Most of the time I try to stay 400 or under or the lowest ISO possible in any given situation. This is one of the major advantages of digital vs film because unless you have a changeable film back for the mechanical camera you can't change ISO where with the DSLR you can switch back and forth with ISO. You have a D300 so you likely have better high ISO performance than with the D5000 me I just have to deal with any noise I get at the higher ISO and the camera has ISO NR but I haven't messed with that yet so I don't know if it really helps. So you really ought to ask yourself if you are going to use the camera to meter and if so how sensitive do you want it to be to the light...it is fairly simple and easy once you get the jist of it.