That's the currently accepted Sema calc, which is 19 megatons or City Level.
The problem as I see it is that Deus Sema from Irene brought a meteor down from space.
I see no reason why Jellal couldn't also have done the same since his move is also "Sema" just without the "Deus."
We have no idea where the meteor would have come from so I will use 7900 m/s, 11000 m/s, and 17,000 m/s values given by SoldierBlue.
It's just a simple number swap. At the bottom of that page switch out the "2000" with one of the three numbers and we get:
298 megatons or Mountain level.
578 megatons or Mountain level+.
1.4 gigatons or baseline Large Mountain level.