The original calculation uses effects from before George Lucas added changes to the series, so I'm going to see if it yields higher results in the updated version.
The reason I'm averaging the two values is because the planet's fragments do not move at the same speed.
Alderaan = 278 px / 12,500,000 meters
Panel Length (from Alderaan center) = 1455 px / 65,422,661.8705 meters
Panel Height = 815 px
2*atan(tan(70/2 deg)*(278/815)) = 0.468902925 radians, or 26.866158603906 degrees
Plugging these results into the Angular Size Calculator gets 26,168,000 meters
(65,422,661.8705 + 26,168,000) / 2 = 45795330.9352 m
The explosion started at 2:09 and reached the edge at 2:11, or 3 seconds
45795330.9352 / 3 = 15265110.3117 m/s
(0.5)(5.972e+24)(15265110.3117^2) = 6.9580845e+38 J
Using the afformentioned site and playing the clip at 25 fps, the beam starts at ~3176 and hits the planet at ~3221, or 45 frames
45 / 25 = 1.8 sec
26,168,000 / 1.8 = 14537777.7778 m/s
Death Star I hits Alderaan = 26,160 Kilometers (Planetary)
Death Star I reaches Alderaan = Mach 42,384.191772 (Sub-Relativistic)
However, they say the Death Star's beams fire at FTL speeds in the Catalyst Novel
- Computer modeling showed the lasers’ twin collimating beams racing away from the Star Destroyer. Then, captured by gravity, the beams become one, changing vector and accelerating beyond lightspeed as it disappeared into the mask’s churning accretion envelope.
Death Star I Firepower = 1.6630221e+29 Tons of TNT (Dwarf Star level)