AWS Simple Scaling - Will it scale in back automatically after scale out?

Question

We are using AWS Simple Scaling policy to scale out our EC2 instance. My company will not consider other AWS scaling method ATM.

 Add 2 capacity units when CPUUtilization >= 70 for 5 consecutive periods of 60 seconds for the metric dimensions

So, we will scale out 2 units when CPU > 70%.

Here are my questions ?

1. Will EC2 scale in back to desired no of EC2 instance AUTOMATICALLY when CPI Utilisation drop less than 70% ?
2. Do I need a separate scale in policy ? If yes, what is the best CPU value ? < 60% ?
3. What is the best practise for using Simple Scaling ?

Also, one of previous workmate implemented an additional scale-in rule like this: Will that working for scale-in ? It seem strange for scale in rule with CPU greater than 15%

Remove 1 capacity units CPUUtilization > 15 for 12 consecutive periods of 60 seconds for the metric dimensions:

Answer 1

Will EC2 scale in back when CPI Utilisation drop less than 70% ?

No, it will not.

Do I need a separate simple scale in policy ? If yes, what is the best CPU value ? < 60% ?

Yes, you need a separate policy. Whether 60% is suited or not, is use-case specific.

What is the best practice for using Simple Scaling ?

I would say stress load your application and monitor how the ASG scales with your policies and thresholds.