I’m sure that there are others who can answer this way better than me but I tried to do some rough calculations comparing Dagon with Relic +3 for Shoha:
Tachi: Shoha is a 2 hit with fTP of 2.6875 at 3K TP and 1.375 at 1K TP [I will do them both]
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At 1K TP:
1.375x2=2.75[Before multi-hits]
Add belt/neck:
(1.375+0.2)x2=3.15
Factoring Multihits:
BiS/Multi Shoha has [ 21 DA
19 TA 3 QA ]
100x0.03=3
100-3=97
97x0.19=18.43
100-18.43=81.57
81.57x0.21=17.13
100+3+18.43+17.13=138.56 attacks
1.39 AVG attacks of a 39% chance of getting a proc of multihit attack with 3.15 fTP at 1K
For the sake of making this easy I will add a constant of (3.15x39%) to the overall fTP(1.23)
Total fTP= 3.15+1.23=4.39 with BiS multi
Now let’s see if you swap Dagon with Relic +3
Shoha with +3 Relic has [ 21 DA
14 TA 3 QA ]
100x0.03=3
100-3=97
97x0.14=13.58
100-13.58=86.42
86.42x0.21=18.15
100+3+13.58+18.15=134.73 attacks
1.35 AVG attacks of a 35% chance of getting a proc of multihit attack with 3.15 fTP at 1K
(3.15x35%) to the overall fTP(1.1)
Total fTP= 3.15+1.1=4.25 with BiS multi
4.39-4.25=
0.14 increase in fTP
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At 3K TP:
2.6875x2=5.375[Before multi-hits]
Add belt/neck:
(2.6875+0.2)x2=5.775
Factoring Multihits:
1.39 AVG attacks of a 39% chance of getting a proc of multihit attack with 5.775 fTP at 3K
(5.775x39%) to the overall fTP(2.25)
Total fTP= 5.775+2.25=8.0 with BiS multi
With Relic +3
1.35 AVG attacks of a 35% chance of getting a proc of multihit attack with 5.775 fTP at 1K
(5.775x35%) to the overall fTP(2.0)
Total fTP= 5.775+2.0=7.8 with relic +3
8.0-7.8=
0.2 increase in fTP
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By keeping Dagon on you lose 10% wsd from Relic +3 in exchange for 0.14 in total fTP at 1K TP and 0.20 at 3K TP which is not recommended imo and I didn't even factor the STP/Overwhelm on Relic +3 in overflow [I could be missing something here and a better explanation can be done by others]
Edit: This is the Shoha Set I used
ItemSet 357738