That gets complicated - formalin is a reducing agent. There is some speculation that the amine-copper bond in copper medications can be broken by reducing agents, that releases toxic free copper into the water. On the other hand, if you remove the copper, you leave the fish vulnerable to ich/velvet. I have not dosed copper and formalin together, so I can't advise you on that.
That NT labs 30% formalin is tough for me to work with in terms of figuring out a dose. We don't have that product here in the US. If their product is truly 30% formalin, then you would need to use about 3x the amount needed for 100% formalin. The basic formula is: PPM times US gallons divided by 264. So - to dose 10 gallons of water with 25 ppm formalin, it would be 0.95 ml of formalin. Since your product is about 1/3 strength, you would add 2.85 ml - check my math though!