That is a pretty good formula but I'm not sure it is completely on point. Definitely good for a ball park estimate but materials come in to play also different poly foams, epoxy or polyester resin, and fiberglass...or carbon fiber if you have money. For example a backyard shorebreak board I made I used some heavy duty 12 ounce cloth and some good epoxy resin and she's about 6'0x20x2.2 and it feels like a 9'0 manufactured board. Still floats very well though.
Also resin application is a factor, my friend was telling me about the new pro boards are being vacuum sealed for excess resin for ultimate weight reduction. Maybe I'll do some research and get back to you guys.
Once again you are talking apples to oranges this is a typical short board regular glassing schedule. It is normal and it is only ball park.
my step up is a 21.xxx with an extra 10 for rubber. more foam never hurt anyone.
maybe for someone your size it doesnt but as a smaller guy I hate too much foam - boards feel corky and i cant bury a rail once up to speed. I've recently had two 5'11" round nose fishes same dims- the one that was 2 1/4" thick is great - the one that was 2 1/2" thick i sold after 5 sessions...hated it..didnt turn.
As far as longboards go i pretty much use Ashton customs because he makes them only 2 3/4" thick for me with pinched out nose and tails...other longboards that a 3" thick or more feel like self guided missiles.