This is one of those things that I thought everybody did, but I've recently been discovering it to be not the case. I use Google Documents to do my grading. Of course, a lot of people use some sort of gradebook online that is set up to keep track of everything. If you do use something of this sort, this will not be applicable to you. But for those that use spreadsheets or just paper, this will make your life much simpler.
My rule is to never, ever write down a grade on my grade sheet that is not in the form it should be. What I mean by that is that if the test is worth 20% of the student's final grade, I will always make the test out of 20 points. Projects or paper homework are easy to do this way as well.
Some things are more difficult to do this way. For example, the online homework that I did this year gave me a final grade out of 100%, but it was worth only 20% of the final grade. So, before I entered the grade, I quickly multiplied it by 0.2 and then wrote in on my paper. And for my reward system, each stamp is worth 2% of the final grade, so I would multiply by 2 when I was checking stamp counts.
Doing this will make your life easier, come final grade time.