The in-line fuse is there to protect the wiring so that current beyond the rating of the wire won't go through it, causing it to heat up to the point of being a fire hazard.
As a general rule, a fuse that is somewhere bewteen the fuse that protects the equipment and the current rating of the wire that powers it will do the job. Personally, I would reconmmend a fuse rated closer to the rating of the fuse to protect the equipment.
I helped install a Carnetx P1900 for someone recently. The Carnetix uses a 15A fuse. We used a 20A fuse on the power wire. So if there is more than 20A on the wire as a result of a short, the 15A will blow first, protecting the Carnetix. If there is still a short, then the 20A will blow, protecting the wire (and ultimately the rest of the vehicle).