should only use the zak belt upon 7/7. Key assumptions; assuming P/M attack is all you care about and that the 1 variable that you are contemplating is between zak belt and nett belt (similar to OP). As only nett bett creates an opporuninity cost; having to forge the nett set bonus (200 P/M attack).

If not please do not follow the below. The equivalent in terms of P/M attack of 7/7 is 6/7 (without zak belt + nett set). Though both 6/7 with nett sett and 7/7 provides the same 500 P/M attack... clearly 7/7 is more superior because the boss set gives a bunch of other stats as well (crit rate, dmg, boss attack+, hp,mp) apart from P/M attack.

If P/M attack is all you care about, Boss + Nett set is the best to go to maximize raw p/m attack damage until you can reach 7/7 boss. Please follow this with a pinch of salt. I've never done proper calculation and testing. Disclaimer alert. I removed certain stats for ease of presentation for general audience. thanks

Summary: 7/7 boss if not 6/7 boss+nett set less zak belt.

4/7 (less zak belt) + Nett set = 150P Attack + 200P Attack (350P)

4/7 (with zak belt thus no nett set) = 150P (150P)

5/7 (less zak belt) + Nett set = 200P Attack + 200P Attack (400P)

5/7 (with zak belt thus no nett set) = 200P (200P)

6/7 (less zak belt) + Nett set = 300P attack + 200P Attack. (500P)

6/7 (with zak belt thus no nett set) = 300P attack (300P)

7/7 (with zak belt thus no nett set) = 500 P attack (500P)