Heart Rate Reserve: Take the desired % of (maximum heart rate - resting heart rate) and add it to your resting heart rate. For example, to use some numbers, if your HRmax is 200 and your HRrest is 40 (you are young and in pretty good shape), then 75% of HRR is 40 + .75 * (200-40) = 40 + 120 = 160 bpm.
Many prefer using HRR instead of just taking a percentage of HRmax, feeling it gives a more useful way to think of aerobic capacity/training. (I believe it is also closer to the more precise VO2Max.)
For any who do not know, note that HRmax basically does not change, other than slowly lowering as you age. The most training can do is slow down the decrease in HRmax. If you see a heart rate greater than what you thought your HRmax was, you now have a more accurate idea of your actual HRMmx -- it did not get higher because of any training you did.
Interestingly, HRmax does vary with the type of activity. It is lower with less weight-bearing exercise.
HRrest, on the other hand, is a function of your aerobic conditioning, and you can lower it as you get into better shape.) It is pretty easy to measure -- wear a heart rate monitor and see what the lowest reading is in the morning before you get up.
As to recovery rate -- your heart is just another muscle. It is well established that you should not work any muscle all-out every day. At most every two days and, as you get older, every three days may be a better idea. Same goes for the heart. OK to do a strenuous heart workout every two or three days. The other days, you should limit yourself to recovery rate, even if that feels pretty slow to you. Less than this rate may be good for general health (e.g. burns calories), but will not do a lot for you aerobically.
Another use is that, if you are in decent shape, you can probably climb that mountain at 70%-75% HRR indefinitely; probably not at > 80% (unless you are an athlete in good training).
Backpacking or hiking for a number of consecutive days does not violate this, because most of us do not push ourselves to our physical limits every day -- that's what long distance racing does.
Disclaimer: the above is generalized, and the exact numbers vary for different individuals. The above is my understanding of how it all works, but I am not any kind of a professional in this area. I know that we have some MD's and long-distance guys in these forums. Perhaps one of them can correct anything I may have wrong, or can expand what I have said.
I just googled for "variation in HRmax", and found an interesting article at http://www.pponline.co.uk/encyc/0186.htm -- does not talk of HRR, unfortunately.
Definitions, including HRR, at Wikipedia: http://en.wikipedia.org/wiki/Heart_rate