Help with RAOBF disc. My measurements are off.

Help with RAOBF disc. My measurements are off.

I’m using real navigation and trying to get a handle on the RAOBF disc, but my findings are consistently about 1500m to 2000m off. I’ve followed video and pictorial tutorials so I can’t figure out what I’m doing wrong.

For example, I had a Nelson class battleship at 2 optics mast height. Actual mast height is 55m. This reads off as (IIRC) 8500m but (I’m testing results with map icons) when I use the map ruler the actual range is around 7000m.

Are there different optic graphics? I realize that optic height can be out due to waterline but not this much.

Anyone have any solutions?