the answer is that each sb had 36(vasari/tec)48(advent) strike craft AND hull of 22500, shields 16250 armour 22.5,
plus the many hangars in the planet.
also I had tried a fleet of 30 antistructuresgarded by 30 each flaks&lrms&repairs, and after 30 minutes the sb was still there.
what I found to work within 10 minutes is 30+carriers, 30+flak, 30+lrm, 5 command crusers,10 antistructure,10 heavy cruisers and 20 light frigates
and some gravwells have more than 1 sb, eg wormholes have up to 3, stars up to 4, homeworlds 2.
would you like to attack a wormhole facing 3 megasb's on standard fleet limits?
harpo