It's recalculated for you every time you hit Save on the weight goals page and the fitness goals page, so if you are changing those settings, that will do it.
Best to work out what those settings need to be, set them, then reset your calorie range once more, and then just leave it.
Or else, bring your goal date forward. The 1200-1500 you want is in line with Spark's minimum of 1200-1550, so if you put your goal date as close as it will allow you to, it'll automatically put you on that pretty much regardless of changes in fitness, anyway.
Deb, in New Zealand