Ilona,
The powder you have will yield the "stock" solution. All the 1+1, 1+3, etc. refer to further diluting that "stock" with water. Different dilutions give different "looks" to the finished product - its not simply a case of less diluted=less developing time (although this is usually true as well, obviously). Stock sloution keeps a lot longer - generally, once you mix a developer further with water, it is only good for that one use and should then be discarded.
Be sure to use very warm water to mix your chemicals as per the instructions on the bag - if you don't, you will have issues with properly dissolving all the chems. Of course, follow the instructions exactly to get the right volume when finished. Of course, be careful when you do this - make sure you don't get a face full of powdered chems. This is unpleasant enough when you mix powdered drinks...
Which leads me to the temperature question. Yes, the temperature most of these times are given at is 20deg C, or 68 F (are you metric? I am, so I'll keep going with metric if you don't mind). You are correct in assuming that you can adjust development times accordingly, and there are charts available at most manufacturers web sites to show you by how much to adjust for each degree over the baseline 20C.
From my experience, I reccomend that you try to get all your chems down(or up) to that temperature as best as you can. This is not that complicated - I use a Walmart thermometer and it seems to do a fine job - if you want to spring for a nice photo thermometer, even better.
If you want to ensure that the temperatures stay constant, use a water bath - place all your containers in water of a known temperature (in your sink, or a basin of some sort). However, I find that unless your house is really hot or really cold, in the amount of time needed to process one roll of film the temperature changes are not very significant to B&W processing, especially seeing as the developer temperature is most important, and that is your first step, so it will be ok if you just make sure you start out at the right temp. The biggest thing to worry about with the other two steps (stop bath and fixer) is not so much the actual temperature (as long as its nothing ridiculous - try to get it the same as the dev to start with) as the temperature difference between the different stages. In other words,
your dev, stop and fix should be very close in temperature or you may have unwanted ill effets on your film's emulsion. This, again, is simpler than it sounds - just set up your starting temperature for all three to be the same, and you will be ok when you get to the end with the amount of time one roll takes. Just be sure that your wash is also close in temp.
I find the biggest pain to be working with stock solutions (like D76 or Acufine) - with Rodinal (which you mix at 1+25 or more), I just run the tap over the thermometer until I get it right, mix all my chems to the amounts I will use, check once more just in case the water temp changed on me, and off I go. With a stock solution, you may have to place it in a water bath of desired temp to bring it to the right temp.
Just in case you need it -
here is an example of a temp/time chart for Ilfords FP4+ (top of page 4). From what I have seen, its remarkably similar for other films.
I am relatively green at this compared to most folks on this site - but feel free to PM me with any questions you may have. If I don't know, I'll at least usually know where to look or whom to ask! Have fun and best of luck,
Peter.
OK - many, many people beat me to it while I was typing my novel

and one more thing - I just noticed you mentioned you will be using HP5+... not FP4+... I did read your post, I just had a brain fart