My advice is to look back over the past week or so, see how many active calories you're averaging, and then set that as your goal. If you find you're not making it frequently, you can lower it at any time. To me, the goal should be motivating not demoralizing, so set a goal you'll usually hit.
As above. I set mine really high for the first week then calculated an average for my actual results. I then added 20 and set that. I do occasionally change it when I'm on leave for example and likely to go on the bike more but I always have my baseline.