I looked at data from insurance rates for World War 1 shipping on various routes. Pre-war insurance rates looked to be about 1.25 to 1.5 percent of ship and cargo. Once the War started rates went all over the place for the first few months, depending on where German surface raiders were operating. However, once things settled down, on the lower risk routes, the rates ran about 3 percent. That probably would work for an Amber Zone visit, and figure that covers just the ship. If cargo is being carried, that insurance would be paid by the shipper, and if speculative, the ship is assuming the risk.
I would therefore recommend 3 percent, but if the ship is on charter or operating for a patron, the patron has to pay that as part of the cost of using the vessel.