fuse for fish finder

I am trying to put together my "new school" power source for a Humminbird Piranha Max 215.  I am not sure what size fuse to use.  I bought the ATM Mini fuse holder.  I would appreciate any input.

 20% more than the draw of the unit is what I have read. I use a 3amp on my Humminbird. Some people don't use one at all. But if you have a warranty problem it may be a problem.

Thanks.  After reading the owners manual 5 times, I finally saw it in fine print: 1 amp.  It probably doesn't make a difference since I can't even figure out how to solder the fuse holder to the battery holder (never soldered in my life).  I thought it would be easy. 

Twist the wires together,flux,then heat the wires not the solder. When the wires are hot enough to melt the solder when it touches you are good to go. Then seal the connection with heat shrink tubing or coat with liquid insulation.
 It is easy after you do it a couple times. A worth while skill to have.