#27 . Even if the amp doesn't draw 250, that inline fuse is there to protect the wire. Unless he meant that's what he WILL be running, even though that's not what he wrote.
OP, the fuses you run close to the positive terminals of the battery should be fused for the capacity of the wire. That's what it is there to protect, not the amp. In most cases it does protect the amp, but that's not the original intention. If you want to fuse smaller specifically for the amp, run another fuse just before the amp of a smaller size. If you are running 2 batteries, you could put that in between your rear battery and the amp. But in between the 2 batteries fuses rated for the capacity of the wire, no more and no less.