First it depends what you want/need to see,
small boats are much easier to detect than crab pot buoys (what is my problem) and how far away you want/need to detect them.
Every boat floats different and as shorter the boats hull is more level changes by different load situations, including the use of outboard and trim tab trim.
I suggest that you measure the level on your hardtop when at the speed you want best reading and that under different load situations and speeds. Have people sit in bow area and then on stern area to see if that changes considerable boats level and if any, consider that as too much downwards with bow high at slow speed will most liekly lead to not seeing very far at planing speed when boats is more or less at level.
A few weeks ago i came back to the marina with 2-3ft short period waves and there was a kayak so i fired up my Fantom 18 and to my surprise the Kayak was clearly visible from about 1/4 mile and at 6 knots, but maybe not at 20 knots when driving home at night?
I fear if you wan't best you need to find out first the average degrees and then tweak it with a wedge or spacers between radar and the SeaView.
If you need to see very small stuff like buoys then i suggest you to anchor a buoy and spend a afternoon to check at what speed and boat level you see her and at what distance she appears and disappears, channel markers or anything similar would work too to get used what your new Radar will show and what not.
Chris