One thing I have noticed with flathead and plastics is that although they are greedy buggers and will strike at anything any size, the hookup rate is much better with up to 75mm plastics than 100mm and upwards plastics. I presume it is because of those gripping teeth in the top jaw getting a better purchase onto the larger plastics and not letting the point dig deeply enough for a good hold.
As a side note I was giving 4-5" plastics a run with a 2/0 jig plus a stinger treble and found that hookup rate was better than using the same plastic with a 4/0 standard jig. Did not really make much of a difference with the size of the flathead caught though with plenty of smaller specimens hitting the bigger plastics.
Had a session yesterday morning wading and caught and released 13 flathead from 30cm to 70cm with most being 40-50cm. I did watch one follow the plastic almost to my feet that would have been 80-90cm roughly. I was using 65mm Squidgy Wrigglers in bloodworm on a 1.5gm jig. Also caught and released 3 flounder and two bream.