How tight?
Having been around International shooting for more years than I want to remember (perhaps I can't remember now!) and I can assure you there is no right way, its what suits you!
I have seen everything at top level from the Gorilla vice grip to the soft holding a very ripe peach grip, and they all work for the person using it.
Thumb pressure can (note only CAN) disturb the hold but not necessarily, if a top level shooter tells you what they think is best its because it works for them, another one will do the opposite, and tell you that's best, and they all will shoot similar scores!
For the record when I shot top level pistol (not now I'm afraid!), I with an air pistol, held firm ish with three fingers, (but no more than holding an egg), but with the thumb relaxed, but the hold was my preference, it does not mean its the best way of doing it, I'd attach a picture of a Korean Girl shooting in a World Cup if I could handle the techy stuff, I'll email it to a couple of you and perhaps you're clever enough to attach it. She has the Gorilla grip and thumb locked down, and its not doing her any harm!
Have Fun
Robin
Walther KK500 Alutec expert special - Barnard .223 "wilde" in a Walther KK500 Alutec stock, mmm...tasty!! - Keppeler 6 mmBR with Walther grip and wood! I may be a Walther-phile?