Here's a little trick I came up with that you might find useful...
COM1 - 03F8
COM2 - 02F8
COM3 - 03E8
COM4 - 02E8
Now, to use the trick, first look and the number after 0. We'll use 02F8 in the example.
This number can tell us if the COM port will be odd or even. In this case, the first number after 0 is 2, an even number. So, the COM port will be even as well, COM2 or COM4.
In the next part we can look at is the letter after that number.
I like to think that "F" stands for first. So, if the letter if "F", it's the first in its odd or even pair, COM1 in the odd pair or COM2 in the even pair. I haven't come up with what "E" could stand for, but by elimination you know it's the second in its pair if it's not "F" for first - COM3 or COM4. In this case, the letter is "F", so we know that this I/O address is first in it's pair.
When we put this information together, we can determine that 02F8 is the first in the even pair, or COM2.
I hope you find it helpful. It seems a bit complicated at first, but once you understand it, it's invaluable.