25 feet up is pretty good. Remember the radar transmitter on a ship is going to be way up there -- 80 feet? 100 feet? So according to the formula [the one in the article Distance to Horizon & Collision Avoidance
-- is it right?], you've got line of sight between a ship's radar and your reflector up to something like 6 miles plus 10 miles -- 16 miles. Even a radar on top of a fishing trawler
should give you something like 10 to 12 miles visibility of your reflector.
Look at it another way: If you put your reflector way up at the masthead, say at 60 feet off the water
, what would you gain? A little more than 3 miles.
So if you move it up only by 10 or 15 feet, it's hardly going to make much difference, and you'll be moving weight aloft, and don't forget windage, both of which are very harmful to your boat's sailing qualities.
I wouldn't bother, personally.
And don't forget that there are numerous reported cases of ships not monitoring their radars and not keeping any decent watch at all, and running down yachts. So plus or minus 5 minutes of radar warning is pretty irrelevant in the grand scheme. In any case you've got to keep a good watch, and make sure your own radar alarms work.
The height of your own radar antenna
may be more important than the height of your reflector -- assuming you get a better radar watch than ships do, as well you should considering how small and fragile you are compared to a ship. Also don't forget an AIS