+T for finding your way here. Using the 20db pad will depend on the volume of the particular show. If you are clipping, you will need to use it. I almost always used it on my Tascam DA-P1, but a lot of current decks have variable input gain, allowing you to effectively reduce the gain level, and then adjust from there with a trim control. If you found yourself clipping a lot in front of the stereo, without using the pad, then you will probably need it for most shows.
Now that you have done the stereo practice run, maybe you can get out to some local club show, that you don't really care about, to test your new-found skills in the field, such as learning to set up in low light (oh BTW, a good flashlight, and a backup, are necessities), and running your gear in a live setting. This will give you more experience, comfort with the gear, and some more information about where you need to set levels, pad etc., although levels everywhere are always different, and be aware that the FOH guy is going to be adjusting his levels at the beginning of the show, usually starting low, and then going up, so you need to keep your eye on things, as you can set your levels, but if the PA gets turned up, you can find yourself clipping.
I just got a new deck in the mail, and I am reading the manual, playing with it, will probably run it along side my existing deck the first night or two, until I feel fully comfortable with it. Good luck, keep asking questions, and read a lot of the posts already here, using the search function.