Currently max reported clear for the top Seismic Slam build will be around 136-138 at 10000 Paragon and 150 gem levels. I think it needs about 5-6GR worth of extra damage buff so that it will land in the ballpark of 143-144 GR (if the balancing needs to be done right)
So
1.17^6=2.6x, which is 1+1.6
So it needs an extra 160% Damage.
Let’s round this up
Fjord Cutter’s damage buff amount needs to be modified to deal 350% increased SS damage (Min Required)
To the attention of developers,
Cheers 
Appendix
If you want to see more options for the buff amounts and the number of GR levels contributed, see the table below. We always welcome higher buffs so if you choose to buff say 10GR, that would be very welcomed. Thanks for your reading.
| +GR | DMG Multi | DMG to be Added | Final Buff Value (%) | Selected Value (%) | Buffed Top GR |
|---|---|---|---|---|---|
| 6 | 2.57 | 157 | 307 | 350 | 142 |
| 7 | 3 | 200 | 351 | 350 | 143 |
| 8 | 3.51 | 251 | 402 | 400 | 144 |
| 9 | 4.1 | 311 | 461 | 500 | 145 |
| 10 | 4.81 | 381 | 531 | 550 | 146 |
Notes and Bugs
Fjord cutter might have a bug. See the thread below. The OP placed a bug report for it:
Bug Report:
https://www.reddit.com/r/Diablo/comments/dw7972/fjord_cutter_bug/?utm_source=share&utm_medium=web2x