The fuse should be smaller than the wire it protects and only needs to be as big as the load you will put on it.
If your only load will be your phone you could use any system you want but only put in a 3amp fuse.
You could carry a larger fuse to fit if you ever have a larger load.
So what ever makes the job easy and as smaller fuse as you need to do the job.
Some of the 19 rides in my shed
2014 V2, Snoopy
2009 Wee, Pumbaa the pig
WR450F, The Blue Postie Bike
YZ250N smoker with rego, Stinky.
Yamaha MT09 (FZ09), The Scud missile.
Club Lead not Club Med.
He with the most toys wins.
Out of my depth in a puddle.
Live life on the edge you will see more that way.
Ridding a motorcycle keeps things in balance.
At the end of each trail and at the end of each day history is made.