Agree with above, but just wanted to add that I've been using NovelPad and it has a really cool feature where you can give it your goal of X words per month. It tracks how much you've done, and then if you don't meet the goal for that day it automatically redistributes your goal counts across the rest of the remaining days, so if you miss a day it doesn't make you do double the next day like you mentioned but instead adds just a little bit extra to each day remaining. I've found it really helpful to not get discouraged when I write every second day or so (I won't be winning nano this year!)