I get what you're saying, maybe it was just an exaggeration of power to hype Broly up and it makes sense via powerscaling. You raise a very interesting point actually so I've decided to do a bit of maths.
Vegeta when he fought Goku had a power level of 18,000 and he was going to destroy the earth before Goku stopped him. Now it obviously doesn't take someone with such a high power level to destroy the Earth seeing as Master Roshi was able to destroy the moon with a power level of 200 and Piccolo was able to destroy the moon with ease with a power level of around about 3000. (Estimates take from the official Daizenshuu guides). If we take the minimum amount of power 200 (which is Roshi's) to destroy the moon and x49(How many times the Earth is bigger than the Moon) we get to 9,800.
So it would take a power level of around about 9,800, probably less seeing as Master Roshi didn't really strain everything into destroying the moon, to destroy the Earth. Once again though this is assuming everything is scaled.
Well when Goku went SSJ for the first time his power level was 150,000,000. Which means he could destroy 15306 Earths. The official Kazentai battle book states that with every SSJ transformation the Sayain's power is doubled. This I assume excludes all the training the Sayain does after achieving a transformation so would probably give a very baseline figure. Either way this means that when Goku went SSJ3 his power level would've been 600,000,000 as a minimum (Probably a lot higher than this). That would mean he could destroy 61,224 Earths.
LSSJ Broly on the other hand was stated to have a power level of 1,400,000,000 which means he could destroy 142,857 Earths.
Now for some size scales. Jupiter the largest planet in a solar system is around about 1317 times the volume of the Earth, so Jupiter would get crapped on by anyone above SS level. The Sun however is massive and it's volume is 1,300,000 that of the Earth.
Hopefully that little bit of quick maths can come in help to someone in the future and hopefully it was a somewhat interesting read lol.
Well you can't actually scale it like that. In order to destroy a celestial body DBZ style, you need to overcome it's gravitational bidding energy. For Earth that energy is 2.24 x 10^32 J. Gravitational bidding energy for Moon is 1762 times less than Earth's. So if we use Roshi as a base for moonbuster than it would mean somebody 1762 times more powerful than him is base(minimal ) for Earth busting. Roshi has a power levels of 139. That means that minimal Earth-buster needs power level of 244 918. So basilcy in tier with first form Frieza (power level os someone like Chilled). You see actuall scietific facts prove that Vegeta in Saiyan Saga wasn't a planet buster. And point is furthere proved since even if he was than it would be a suicidal move since he can't survive in space, and why would he kill himself ? Anyway to get to the minimal star buster you need someone 1 000 000 000 000 times more powerful. Someone with power level of 244 918 000 000 000 000. And even power levels for DBM (until they took them down) weren't that high. For example SSJ3 Veggeto with Final Dragon Flash had a power level of 5 000 000 000 000 000.
And even if we aply your calculations we won't get any star buster (with official power levels from V-Jump). For example SSJ Gogeta had 2 500 000 000 and he still wouldn't be star buster even with your calcs. About Vegetto:
Contrary to what everyone belives, Old Kai didn't stated that POTARA MAKES STRONGER FUSION. He only stated that it's BETTER THAN FUSION DANCE (i posted this in caps to be better for seeing). And he surely ment better because POTARA is permanent unlike dance which lasts for 30 minutes. Anyway in my book Vegetto = Gogeta in power :-). But Vegetto would win in fight since Gogeta is limited to 30 minutes.
About official power levels for Broly and Gogeta i say that they are to low they should be at minimum 10 times greater (even 20 times for Gogeta). Official power level for Cooler 470 000 000 seems little too high. Anyway i've gone of topic too much. In order to check about gravitational bidding energies you can check Wikipedia or net. I can't post links since Rules forbid it. And i broke that rule once today. Since i'm new i don't want to break it twice, mods might get angry.
Sorry for the late reply, flood protection prevented me to reply last night.