I might get bad responses from this.....But this is what I think:
Obviously the bigger the better for a Dovii, so if you can provide a 30, 36, or deeper tank, that is awesome, and you should do so. It'll only be that much nicer for a Dovii.
But you'll be fine with a 6X2 (standard 180) print for life for a single male. An 8X2 (standard 240) would be even better for the extra swimming length.
To say that you'll be able to "get away" with a 90X40 footprint is a bit misleading. That would be a GREAT size tank for a Dovii. You won't just "get away" with it, you'll have a VERY happy Dovii in such a large tank.
There's just not a ton of males that get as big as Mel's, and most will max out around 20-22 inches. So I keep to the opinion that a 2' deep tank will be fine.
But to each their own. Everyone has a different opinion on what is necessary to keep one of these. An you do have to keep in mind that these guys are going to make a crazy large amount of mess in a 180 or a 240, and you are going to have to have a really big filtration system, and are going to have to do a LOT of BIG water changes. This is where haveing a bigger tank comes in really handy.