Delayed response.
(Didn't read the entire thread)
I was using the same tactic as you before, but was told that an important of maaser is to actually separate money in a calculated fashion.
Just to break this down a little for CV's benefit. There is a difference between maaser and general tzedaka. Maaser is often translated as "tithes" which is the obligation one has to give 10% of their earnings to charity. As this is an obligation, there are various rules in play, such as what is considered a valid charitable cause, etc. Tzedaka is translated as charity (this is actually a mistranslation, but that discussion is for a different time), and that would include any charitable donations, and there are no rules attached.
So for the maaser, it is important to calculate the correct amount and keep track. Some make sure to be so exact as to not round up. But for tzedaka one gives on top of that, one need not be so exacting or keep track, and many will be sure to give generously without keeping track exactly.