I often don't call out enemy CV when I find them and have a PT boat in good position for a run.
If you're in front of the enemy cv or within 30 degrees of head on, your torps that run for 4 minutes can get to the fleet with the combined speeds building to about a 70mph closure rate.
So you can launch before the cv flashes, turn immediately around and drive straight away to maintain distance.
I usually drop one for the cruiser and 3 for the cv.
If you want to try for 2 cv and 2 cruisers, you have to get within 40mm gun or rocket distance which flashes the cv.
The biggest obstacle to success in using PT boats on enemy cv groups is your own countrymen since they will cause the cv to turn making your torp runs useless as they spend themselves flaccidly against the hull on 13 one way missions just to light it.