Owning the same boat for 18 years gives me a feel for the costs. Basically the percentage rule doesn't make any sense to me. In a year in which there are no major repairs or additions I spend $10,000. I do much of the work myself but I do bring in outsiders for several things.
Over the two decades I have added major items, watermaker, stabilizers, dinghy crane, wing engine, new dinghy, new fuel tanks,generator rebuild, built in furniture. All of these have driven that year's cost through the roof. These costs appear to be in no way related to the value of the boat over the years.