Battery health 90% after 350 cycle

Apple's documentation states that the battery in the 16 Pro Max was designed to go up to 1,000 Charge Cycles before the Battery Health / Maximum capacity declines to 80% under ideal conditions.

That works out to about a 1% decline on average for every 50 Charge Cycles.

So, under "ideal conditions", the Battery Health would be expected to be in the 93% range after 350 Cycles. Unfortunately, Apple does not tell us what "ideal conditions" might be, likely because folks don't all use their iPhones the same way.

90% is not bad, so nothing to be really concerned about.

All that you really need to remember is to replace the battery when the Battery Health dips under 80%. Your battery is about half of the way to that point.

Some good info in the document linked just below:

When to charge your iPhone or iPad - Apple Community

While your battery health has declined slightly faster to 90% after 350 cycles, I wouldn't be overly concerned about it. Read this and pay attention to the links about how to keep your battery healthy --> Battery Health Decline - Should You Worry… - Apple Community

Batteries don’t age in a linear fashion. Some aging comes just from time. When was the battery manufactured?