A promising app, and a useful program concept, bit I can't help but notice the Accumulated Surplus calculation is occasionally way off, as is the remainder of month calculation. I've been testing SurplusMeter v2.0.3 since January, and I found that these figures are unreliable, based on the way they are calculated. As an example, I started a usage cycle yesterday, and during the day used ~300MB less than my daily allotment. Today, while still more than 400MB below my daily allotment, the Accumulated Surplus reads -45.3MB, and the "remainder of month" calculation is in the red, even though the average daily usage is just over half of the daily allowance. The "remainder of the month" calculation assumes that the current day has not yet begun and merely divides the total MB available for the month by the number of days left, where it should take into account the % of the current day already over and how much data is being used daily. In other words, this figure is only really accurate between midnight and the first byte of network activity of a given day, and approaches a greater degree of accuracy the more days go by. The same applies for the accumulated surplus; it assumes that all data transferred so far was a part of the preceding day(s) allotment, and doesn't take into account that some usage was from today or that there was an allocation made specifically for said usage. The system draws from the surplus first instead of the daily allocation, making it look like I'm in the red when I actually have hundreds of MB left.
To illustrate: I assumed a stringent cap of 25GB for the month (31 days). Of the 1651.6 MB allotted for 2 days usage, I have used 871.3 MB, which should leave an available reservoir of 780.3 MB before it impacts future daily allowances negatively. The current calculations make it appear that I'm far in the red when I'm actually substantially in the black (or green as the case may be). Clearly, if I'm consistently using less than the daily allowance, the surplus should be close to an entire day worth of usage (or should only reflect the left-over from the previous day(s)), and the daily remainder for the month should be higher than the overall daily allowance.
Needless to say, the app has limited usefulness as a result of this method of calculation. Blissfully, by the time I reach the end of a month, the calculations will average out a bit better and approach some semblance of reality, but until then I have to do my own math to predict the impacts of my usage trends accurately.
I recommend reconsidering the calculation method to better reflect ongoing usage. For starters, not deducting from the surplus until the daily allotment has been exhausted would be a good start. Unfortunately, to accomplish this the system will have to keep a record of the usage at the precise beginning of the current day, and this may be impossible given the way SurplusMeter currently functions.