If 20% of the 2019 Clacton Tories switched to Labour and half of the rest went to Reform, Labour would win. That’s without taking into account those that won’t bother this time which will impact the Tories more.
Survation did some hypothetical polling (which is obviously very shaky, but apparently was a private contract aimed at persuading Farage to stand). Unfortunately it suggested Labour's vote in Clacton could actually go down slightly if Farage stands.
Bear in mind, Labour's vote has gone up quite a lot since the last election, and some of those will be the social conservatives/leavers that Boris appealed to, and that Farage is may be able to attract.