Ok, I think that it's time to make an accurate tier for Future Arc characters. So I do this
Feat[]
Tsuna's X-Burner vaporized 3 blocks of Melone Base.
Calculation[]
First, I need to know te size of each blocks. Luckily, I can easily find it with Melone plan below.
The Melone Base has 300m x 300m x 1000m on size. The plan shows that it has 7 blocks on both width and length.
So, block's width = 300/7 = 42.86m = 4,286cm
The block is a cube. So, Volume of block = 4,286^3 = 7.8717201e+10 cm^3
I assume that the block has 80% of hollowness.
So, real volume of block = 7.8717201e+10 * 0.2 = 1.5743440e+10
X-Burner is a powerful flame attack. We can also see on feat that vapors come from everywhere. So I use vaporisation(25700 j/cc)
Energy for vaporized a block = 1.5743440e+10 x 25700 = 4.0460641e+14 joules
Tsuna destroyed 3 blocks of this. So I multiply result with three.
Finally, Tsuna's X-Burner = 4.0460641e+14 x 3 = 1.2138192e+15 joules = 290 Kilotons of TNT (Large Town level)