best time to port out number

That is correct. The amount will be prorated based on the cancellation date.

The best time to port out is on your last day of your billing cycle, so you will not lose the auto-pay discount.

So it can't be April 11 to May 12.

If the billing period starts April 11 then it ends May 10.

If the billing period ends May 12, then the billing period starts April 13.

So you need to make sure you have your billing period correct. 

Now to avoid any unexpected charges, it may be unavoidable with how koodo sets things. 

-If you cancel 12:01am on start of the new billing day you will be charged for prorated 1 day (no auto pay discount applies, but if it is given it will be reversed on the next bill) 

-if you cancel 11:59pm on the last day of your billing period, koodo may (and this is a big maybe) reverse the auto pay discount as it technically applies to a full billing cycle and I'm sure if koodo will consider this not a full Billing cycle 

Good catch Dennis. I got it backward. The bill was from April 12 to May 11.
If it is true that a $10 fee is charged at the time of closing a postpaid account just because the Auto-pay is part of that process, the it deserves some push back. The Community says a monthly account is automatically closed when its number is ported out. It might be understandable if the final bill had to be collected by other means than Auto-pay. But all of my bills were paid in full by Auto-pay justifying the discount every time. If Koodo sees a need to send another bill after my account is closed then feel free to withhold the discount, but sending an additional bill just because I erase my banking information when I don’t owe for any services rendered. Well, we will see how that goes. 

I am referring to credits only applying to full billing cycles. 

Reports on this community is that if you cancel before a full Billing cycle, the account is charged back the credit. 

https://www.koodomobile.com/en/terms/service-terms