Ok guys, so after a couple hundred g's worth of gil and frustration over screwing up my desynth allotment and trying to redo my points constantly cringing at deleveling the lvl100 skills that I worked so hard on, I think finally found the trick (with the help of FC buddy Serafina Faye from Adamantoise) to control what desynth craft get deleveled first.
*Disclaimer*: This is guide is more geared to those who already hit the 300 level cap and want to shift some points around. Those who are getting screwed over by the -0.01 RNG might not find this to be much of use unless the leveling mats are too costly to afford a random delevel, then you should apply this trick as a safeguard.
Let's go straight into a typical scenario of how you want to control your deleveling:
-You maxed out two crafts A and B to 100
-You have two other crafts C and D sitting around 45~50 ish
-Now you're most likely already maxed out your 300 allowed desynth level, but you really want to get a 3rd class up to 95 so that you have 3 useful desynth classes.
-Say you decide to pick C to max out and want to scrap D, each time you level up C, you will randomly lose skill points in A, B or D to balance the total back to 300 (yes, losing 0.1~0.5 on a lvl100 skill IS PAINFUL)
Here's what you would need to do to level up craft C without messing with your "perfect" A and B crafts, while deleveling D craft:
-First, know that a desynth craft will NOT delevel if it is flagged as a "active". From my experiment, a desynth craft will be active for your next 4 (see note at bottom for reasoning) SUCCESSFUL desynth rounds, including the round that you used to flag it as "active".
-To save cost, you will want to go to an npc and buy a cheap item and desynth it on the craft that you want to lock. In our scenario, we want to flag A and B as active first to lock them before we try to level up C. Do it in any order you want, A then B or B then A, doesn't matter. I would stick with one rotation consistently for simplicity.
-Now that you locked the two crafts you like, you can begin desynthing to level up your craft of choice, which is C. This should automatically delevel craft D in return to balance the total to 300 again. It is safe to desynth UP to 2 TIMES to level up like this.
-Repeat the whole process again every time you want to level your C desynth.
Here's a short snip of my technique working. In my case, I lock Blacksmith and Goldsmith first using cheap npc items, then level Armorer, while deleveling Carpenter (yes I made a mistake and mixed the order on one of the lock but this shouldn't matter if you know the limit and only try to level up for 2 rounds):
Further finding: It might even be that the maximum active round is up to 4 instead of 3 like I thought, so in my example, I can do: BSM lock > GSM lock > ARM lvl > ARM lvl > repeat to allow an extra desynth and save on spending gil on npc items. If i try to do an extra ARM lvl instead of repeat, it seem that my BSM skill will lose the lock and delevel (most likely because It was locked the longest and expired). Conversely, If I try to do GSM lock before BSM in my rotation, then GSM will be the one to delevel first instead. Food for thought.
Follow-up: After another desynthing another batch of items for another hour, feel that it's safe to confirm that the maximum locked round is a maximum of 4, including the desynth round that was used to lock a craft. Guide is updated to reflect changes.
At the moment I don't really have time to test other scenarios and I am tired of spending more gil to test this and I will just stick to this method to safely cap my 3rd desynth. If you guys find something better let me know so I can update. Lastly, sorry if I am not good at explaining and writing guide but I hope it is enough save you guys lot of gil and headache.