Note: This Calculation was originally done by Vivi Ornitier from OBD.
The feat in question can be seen here(jump to 2 minutes).
So Setsuna while pilotting GNT-0000 00 Qan[T] is able to extend his sword thousands of kilometers and scar the moon-sized ELS Planetoid.
Will use 3000 km for a low end and the moon's size for a high end
First off, the expansion speed[]
726/688 = 1.05523256
meters extended 1.05523256 * 3476280 = 3668283.84
It takes only 2.74 seconds to extend.
1338789.723 m/s or mach 3903.177
For Low End
meters extended 3000000 * 1.05523256 = 3165697.68
1155364.117 m/s or mach 3368.41
Now for the damage dealt[]
502/688 = 0.729651163
meters for the length of the sword's penetrated volume 3476280 * 0.729651163 = 2536471.74
62/2744 = 0.0225947522
meters for the width 3476280 * 0.0225947522 = 78545.6852
Not sure of the thickness since the shots are nigh-impossible to determine accurately but the walls' radii are at least equal or so to Brave's height which is 20.2 meters.
2536471.74 * 78545.6852 * 40.4 = 8.048848e12 m^3 or 8.048848e18 cm^3 sliced
Violent fragmentation of military grade steel = 1000 j/cc
8.048848e18 * 1000 = 8.048848e21 joules or 1.924 teratons of TNT equivalent so Small Country level Setsuna
For Low End
meters for the length of the sword's penetrated volume 3000000 * 0.0225947522 = 2188953.49
meters for the width 3000000 * 0.0225947522 = 67784.2566
2188953.49 * 67784.2566 * 40.4 = 5.99441404e12 m^3 or 5.99441404e18 cm^3 sliced
5.99441404e18 * 1000 = 5.99441404e21 joules or 1.433 teratons of TNT equivalent
Final Tally[]
00 Qan[T]'s GN Buster Sword Extends: Mach 3368.41 - 3903.177
00 Qan[T]'s GN Buster Sword Scars ELS Planetoid: 1.433 - 1.924 teratons