Hey there HenryA
If your Nord has balanced outputs (Tip/Ring/Sleeve, "TRS") I would use those directly into the FF800 with 1/4" cables.
I have my FantomX7 with 1/4" TRS balanced stereo outs going into the TRS balanced inputs of my FF800 on the front inputs 7/8. I adjust the gain control so the LED's don't turn yellow with my Fantom volume level at about 75% (2 o'clock) and the gain knob of the FF800 at 50% (12 o'clock) and it sounds great! Using a DI would just be overkill. I usually reserve the instrument input #1 on the front for guitar or bass input...
You can also use the rear inputs just as easily. Check your FF800 setting dialog box to make sure all the settings are correct and record away...I love the FF800 and think it is a fantastic interface and it sounds amazing....
If you read over the manual, Pg55 explains quite a bit about the analog inputs and outputs and levels etc.
Hope this helps!!

Oh thank u.
Yeah my Nord have unbalanced outputs, thats why i use unbalanced instr. cables... But FF on the other hand have balanced inputs so im not shure is that a problem.
Oh yeah and i conected it to back inputs. But i can see why is front alot simpler. Idk why i didn't conect it like that in the first place...lol

And yeah i readed pg55 but i must admit i can't tell any difference when its on +4Dbu or -10dBV...

The Nord has unbalanced outputs vaguely at domestic levels (-10dBV). You are correct to use unbalanced (TS) screened jack cables into the FF800 rear panel. The TS cable will correctly ground the ring connection (-ve) of the FF800 balanced input. You do not need a DI box for this job.

Thank u very much