I've heard the optimal level for ZMI is 75 so wasn't sure if it's still worth doing at these low levels; just need it for the 196 air rune task of the hard Falador diary. Any info is appreciated

Those levels are where you actually stat to make profit. Before those levels, the mid 60s, you about break even. Below those levels you lose some money on the runes crafted. It still is decent, and fast exp. Smoke and Steam runes are fast as well whereas Smoke make profit but not as much exp and Steam make tons of exp/hr but you lose money.

 

 

 

I personally like ZMI. I got to 61 in hopes of getting to 66 for the diary. I was getting about 620 exp a rune. Comparing that to other methods (double cosmics, nature runes, and laws) it seems to be faster but not as great a profit.

I see. Is there any way to calculate how much essence I'd need for those levels? (it's 345k xp)

I doubt it. But at 60-61 i was getting 620 exp a run, i said rune earlier but that was a typo. Those levels are about midway between yours and your goal. You could use that as an average to figure it out and get you a place to start. What i did was i bought 10k pure essence and just crafting most of it. I was at 56 and about 8-9k ess got me to 61.

 

 

 

I used a Small, Medium, Large pouch, one spot for fire runes and the rest was Pure essence. I was on Lunar Magic and i banked with the banker in the ZMI area.