Expansion Jumpstart Guide

[Ultimatecalibur]

Frontloading or not, completing the challenge log before EA gives zero bonus. 10% of 51 vs 10% of 52 is a small difference, and having that extra level gives more exp for new dungeon mobs, FATEs, etc. Holding off for a higher level is robbing Peter to pay Paul.

I wasn't advising against leaving the Challenge Log challenges unfinished before the 19th, I was advising against leaving them nearly complete and then using a roulette to finish them off.

You've got your gains backwards, holding off on the percentage based rewards will mean a faster trip to 60 not slower as much of the initial Combat Class XP will come from completing the new Main Story quests.

I currently have 16 DoH leves set up with the stuff already made, along with the mats for 8 more at St Coin. Those 24 combined will get me roughly 480k exp on all 8 DoH jobs. 16 free leves > 0 free leves, regardless of how much exp they give.

The 16 leves that you have already prepped? They will give you a slight jumpstart on leveling the DoH classes, but it is likely to be far less than the 480k you are expecting as the "easy job" penalty kicks in.
Those 8 Leves that you've already prepped Mats for? Those 8 will likely be giving 4 times as much XP/Gil/Seals if spent on level 50 Ishguard leves instead of level 45 St. Coin leves.

The best gains from prepping will likely be from prepping 16 level 30 to level 45 Combat leves if you want to get the 3 new class to 60 asap.