I was bored. What do I do when bored? Math, apparently. Went over a lot of scenarios where 1k fuse is possible, as well as how high it can get in general. If I inspire any try-hard Musicians, I've gone too far this time.
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In order to heal at least 1,000 MP with Transfusion Tune (and your target is NOT a Shaman with Ying-Yang Transform), you need at least:
Level 12 Transfusion Tune (650 base MP restored) <--it MUST be at this level
True Insturement - +20% <--manditory
Physical Gold Insturment - +20% <--manditory
Level 7 Soul Music - +14% <--MUST be at this level assuming you do not have any Musical Insturment Skill Charm
This should result in 1,001. Oh dear, odd numbers. Anyway, for level 12 Transfusion Tune and level 7 Soul Music, you will need skill scrolls for them. You will also need a skill scroll for Mystic Mantra level 16 since you need that to get lvl 12 fuse as well. All of this can be done while you're still level 70. However, you must also be level 72 for level 7 Soul Music. The physical golded true insturment is achieveable at level 65 though (True Secret Spike Zither anyone?).
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That being said, can you do this on another level 65 support job (like Doctor), assuming you ARE a level 72 Musician? Keep in mind Soul Music's effect is halved on other jobs.
Level 11 Transfusion Tune (575 base MP recovery restored) <--The reason we use this level is because level 12 Transfusion Tune's penalty is counted as if it were a level 73 skill, meaning it would only have 60% of its effect if you were level 65. Therefore, 11 is better (and safer).
True Insturment - +20%
Physical Gold Insturment - +20%
Level 7 Soul Music - 7% (since you are not on Musician)
Divine Musical Insturment Skill Charm - +10%
Assist Bug (Soul Music, if you get another Musician kind enough to use it) - 4%
With this, we can only get 925 MP restored. What if you're using it on a Shaman?
Level 5 Ying-Yang Transform - +10% <--Cannot be lower than level 5
Assist Bug (Ying-Yang Transform) - +4% <--without this, the Shaman in question will need to be level 72 for level 7 Ying-Yang Transform
This should result in 1,006. But this is only if another Musician and a Shaman use their Assist Bugs, while on a level 65 non-Musician support job (Doctor or Dancer). If you can't find another Musician to do this, then the Shaman will need level 7 Ying-Yang Transform for that missing 4%.
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Now you for the big question: How high can Transfusion Tune recover?
Level 12 Transfusion Tune (650 base)
True Insturment - +20%
Physical Gold Insturment - +20%
Level 7 Soul Music - 14%
Level 7 Ying-Yang Transform - +14%
Divine Musical Insturment Skill Charm - +10%
Assist Bug (Soul Music) - +4%
Assist Bug (Ying-Yang Transform) - +4%
The answer is...1,209! If you want to see that number, you and a friend need to rebirth! 8D On a non-Shaman Target? The answer will be 1,092.
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Is 1,000+ fuse possible without ever having to rebirth? You're only losing 2% (4% in total if we count Shaman), but...
Level 12 Transfusion Tune (650 base)
True Insturment - +20%
Physical Gold Insturment - +20%
Level 6 Soul Music - +12%
Level 6 Ying-Yang Transform - +12%
Divine Musican Insturment Skill Charm - +10%
Assist Bug (Soul Music) - +4%
Assit Bug (Ying-Yang Transform) - +4%
The result is 1,183, so yes, even with level 11 Transfusion Tune (without assist bugs, it would be EXACTLY 1,000 since DoMO likes to round down!). But on a non-Shaman target? The answer is still yes, but you will defintely need a skill charm at that point, and your fuse will need to be level 12 again.
Level 12 Transfusion Tune (650 base)
True Insturement - +20%
Physical Gold Insturment - +20%
Level 6 Soul Music - +12%
Non-Divine Musical Insturment Skill Charm - +5%
Answer: 1,020
So in short, if you ever want to hit the big 4-digit fuse on a non-Shaman target, level 12 is a must, along with a physically golded true insturment and a Musical Insturment Skill Charm (divine or non-divine) unless you don't mind rebirthing for level 7 Soul Music. There should be enough values in this thread on skills and items that you can refer to if you want to see how much your own Transfusion Tune will do. I hope there were supports out there who were just as curious as I was.
Note: The % values are additive. After adding the % values, convert it into decimals and add 1. Then multiply that number with Transfusion Tune's base. (Following the last example: 650*1.57)
Small shoutouts to Aiya for hitting me with 1,196 fuses and Tiriel for discussing the possibility with me a long time ago even though neither of them play much anymore.