Can I ask, how do you determine the amount of K meta to add? Is it just a set amount per gallon (1/4 teaspoon in 6 gallons) or are you testing the wine and dosing accordingly?
It depends on if you are using KMS for oxidation or for microbial stability. For microbial stability in dry wine you would need a molecular of 0.5ppm for bacteria and molecular for 0.8ppm for yeast. This is derived from your free SO2 and your pH.
Lees will scavenge O2, SO2, as well potentially some colour depending on yeast strain used. It also provides food source for other microbes if wine is not microbially stable. Otherwise fine lees contact is philosophical, some winemakers will rack dirty post primary and take gross lees and others will use MLF lees.
e.g. Barrel fermented Chardonnay is left on ferment lees as well as MLF lees if MLF is conducted. Then it just depends on taste.
For reds, it is quite common to have say 2 - 3 litres of lees in a barrique before the first racking.
If you are using the the potassium sorbate for microbial control then you dont need as much. 20ppm should be plenty.
1/4 teaspoon is roughly 1.42g of KMS --> 0.8094 Sulphur.
So in 22.71L (6 gallons) this works out to be 35ppm of S addition. Rough conversion is between half of that will be bound so say you have roughly 17ppm of FSO2.
I don't have any experience with how much FSO2 get bound in a carboy over time in a temperature controlled environment.
In barriques it is accepted in a temp controlled cellar you lose about 5ppm per month.
Oxidation in wine just smells like sherry, so if the surface of your carboy smells like sherry you can assume you're running a little low on FSO2.
The legal limit for commercial wine in USA is 350ppm total SO2 (free SO2 + bound SO2). Normally you wouldn't get anywhere close to that. If you aren't sensitive to sulphites you will be alright if you sneak higher anyway, legal limit in desert wine is higher again.
For reference the legal limit for SO2 in dried fruit in Australia is 3000ppm and in EU is 1000ppm.