There's a lot to consider when setting and adjusting MACS, here is a quick list of what to do and more importantly, what not to do.
What to do:
Set MACS based on campaign goals and where the products are in their lifecycle: profit vs launch, seasoned product vs new product
Consider category competition: higher MACS are needed for categories with lots of sellers
Set MACS no lower than 10% of campaign ACoS
Review MACS (and bid change history) every 2-3 weeks, making changes where necessary
Set the "Bidding Strategy" in Seller/Vendor Central to: 'dynamic bids - down only'
Be patient. Data collection takes approximately 30 days, for low volume sellers, expect this time period to be longer
What NOT to do:
Do not set MACS more 10% lower than current campaign ACoS
Do not make frequent changes to MACS: more than once in 1-2 week period
Do not reduce MACS more than 10%-15% at a time
Do not reduce MACS during the discovery stage
Do not use Bid Floors for an extended period of time (over 2 months)
Do not set MACS below 40% for newly launch campaigns (no data)
Do not set MACS on campaigns that house ad groups with different goals/objectives
Do not make manual adjustments to bids in Seller/Vendor Central
Do not set Bidding Strategy in Seller/Vendor Central to 'dynamic bids - up and down' or 'fixed bids'
Do not use 'Adjust bids by Placement' booster in Seller/Vendor Central
Next: Using the MACS Calculator