How many perk tiers for prestige conversion?

I have 2 survivors that have exactly 60 perk tiers unlocked, prior to the anniversary event. And they did not receive any prestige in the PTB.

Then I have bubba which has exactly 240 perk tiers unlocked (this one, all by level 3 perks). And he received level 4 prestige.


So how many perks tiers do I need to unlock to guarantee a prestige?


For reference, i got with other characters:

Prestige 1 with 74 - 114 perk tiers

Prestige 2 with 139 - 165 perk tiers

Prestige 3 with 190 - 203 perk tiers