I dunno. The raid by the PoR did around 140b in damages, as far as I can tell, and required nearly the entire fleet (built with 3x the budget) to put down, and as far as I can tell from my non-GM seat, it not being a full success was down to dice rolls. Ultimately, we can not stop an enemy from just appearing in a system of their choosing, so the best defence would be to make sure they don't choose that system.
Of course, stations are the more obvious application, but those have a running cost.
In fact, it seems to me that doing deep strikes at nothing in particular may be a way to go. You'll cause a lot of economic damage while avoiding most opposition, as such opposition can't afford to leave the actually valuable stuff undefended.
It seems to me that if the enemy is faffing about in my space with a few ships, I take a few more ships and go kill them. Not always easy, but possible.
If hes running around in my space with most of his ships, but wont hit hard targets so Im not rushed, I take almost all of mine - and now hes out most of his navy, and I go visit his capital and set terms
If he brings his whole navy, and somehow ISNT trying for a KO - well, then, Im definitely off to visit his capital.
Sending a lot of your navy off deep into enemy territory is incredibly risky. One reason (and there are many more) is the information advantage the defender has. Every time you hit a system (and you are hitting something, else you would not be here), you tell your opponent roughly what you have and where, and you probably don't know what he has and where. Now, a true ‘bolt from the blue’ attack, where you never enter an inhabited system until you jump into the capitals closest pirate point helps SOME... but who is minding the store at home?
As to dice - the single largest dice roll was the defending admiral expecting that they were aiming for the nearest shipyard - because skipping it to go to Luthien was a lot more risk for a little more reward, and then going on to New Samarkand from Luthien was basically insane - so he didnt expect it - so he was out of position. Had he properly anticipated what the PoR was thinking, hed have fallen back to Luthien and met them there with two fleets, likely ending the expedition there.
But 'enemy capabilities and intentions' is a game that everyone always plays, but one that noone reliably wins.