Hey glad to help
I always put a fuse holder on the power wire even if there is a fuse in the back of the radio. Just a little extra insurance
As for the amount of current, I am not 100% sure of your question, but this may help:
If the place you hook up the line to on the fuse box has current only when the ignition is on, you'll only have power when the ignition is on.
If the hookup always has power, you'll be getting power all the time no matter if the ignition is on or not. I ran many batteries down by doing that and forgetting to turn off the radio when I left the vehicle
If you mean like amount of electricity, you don't have to worry about it being regulated. I expect that this might fluctuate on a vehicle's electrical system, but not much. I have never had any problems with surging or anything.
Question...does this new radio have multiple power wires? One might be yellow. If it does, you probably have a radio that saves your preset stations, has a clock, etc. If it does, you'll want to run that yellow wire (could be orange also, I think...) to part of the fusebox that always has power (and use a fuse holder on this also).
One thing that has been VERY handy for me is an automotive circuit tester. You hook up the alligator clip to a grounded surface (some place on the body generally...may have to try different areas to get a good ground), and put the point on the place you want power from. It seems like every fuse box I have ever dealt with had the letters telling you what that fuse did worn off. It really helped me.
If you are unsure if something has power all the time, or only with the igniation, use one of these testers and switch the ignition on and off. It will light up when power goes through it. A picture of one is below
Sorry for being so long winded, or if I have went over stuff you already know. I just know how it was when I first started hooking up stuff, any information was helpful!