There are some fights where lusting late is beneficial (Chimaeron / Maloriak / Ascendant Council etc.). But, if you are not attempting these bosses, it is beneficial to lust early. (read: early = lust buff finishes before 35%)
Think of it as intro calculus.
Basically, lusting early reducing the time that 100-35% takes, and lusting late
reduces the time that 35-0% takes.
Let us say that players will do 15,000 dps during non-execute phase. and 17,000 dps
Now we take the limit of DPS as time 100-35% approaches 0. We expect to do 17,000 dps
for the whole fight, since the whole fight is now just execute phase.
Alternatively, if we take the limit of DPS as time 35-0% approaches 0. We expect to do
just 15,000 dps, since the whole fight is now just regular phase.
With all the classes that have "executes" (fire mage <35%, shadow priest <25%, dps warrior <20%, assassination rogue <35%, ret paladin <20%) your raid will benefit from maximizing the time that these executes can be used, even if the actual dps increase from an early lust is very small.
Feedback always appreciated,