tl;dr: Data seems to show that Foil decays about 3 evasion every tick starting at 150 evasion. This evasion only applies vs mob TP moves and is not visible in /checkparam.
Data (first 3 columns, they are time in seconds since cast, hit/miss and the message ID of the attack): https://cdn.discordapp.com/attachments/783509444595023902/976984027028418570/excel_abuse.xls
addon used to gather data for columns A/B/C:
https://pastebin.com/v4RXsrhS
Setup:
Tested with mastered RUN (0 ML) with enhancing skill of 440 with gear adjusted to have an approximately 89% hit rate from Jump from an orc named Draketrader Zlodgodd in La Vaule. Empty slots were required to do this, so my effective ilvl was 113. Enhancing Magic Duration+ was 65% total (including job points).
Draketrader Zlodgodd was level 83 every time I pulled it (At least, that's what the widescan packet claims.)
The testing pretty much was just recast foil before it ran out and just collect samples for an excruciatingly long period of time. The addon above was used to mark the local unix time of when I gained foil, and to log the delta time between when I gained foil and when one of the 2 jump TP moves hit me. Gear was never swapped.
Now onto the good heavily mathy stuff:
I normalized the data into ticks (delta time of 0-2 = tick 1, delta time of 3-5 = tick 2, etc)
You can see there's a very nice correlation line there:
so I truncated it to 4 ticks and mspainted it from zero...
1 acc = 0.5% evasion, so it looks like Foil loses about 3 evasion/tick.
there's about 4 ticks worth of floored evasion so:
89%-20% = X (acc delta to floor)
X + 1.5% * 4 = 75% = 150 evasion at delta tick 0.
Of note, the decay lower limit is not yet reachable as of this JP devpost:
RDM will probably change that. I also do not know if there are any "tiers" to Foil, I just dont have enough gear to do that. When Foil is usable as /RUN we will probably know very quickly if this test is repeated on a job that doesn't have Enhancing Magic natively.
Any feedback on the data interpretation? Thanks.