TOPIC: Discounts calculating in different orders

a minor anomaly in the way discounts are calculated:

a discount code discount is subtracted after the early registration discount is applied. ex: $100 - $10 - 25% = $67.50

a custom field discount is subtracted before the early registration discount is applied. ex: $100 - 25% - $10.00 = $65.00

How can this be corrected to always follow the first method?

YOU control the ordering of calculations. In the event management area, click the "Fee Ordering" link. Then you can drag-n-drop items to set the calculation ordering.

that is not an option in 2.7.18 that I can find.

In backend event management, you will find it. It was not added to frontend event management until 2.8. It's just in the backend list of events... in each row that shows an event, date, etc... towards the right is a "Fee Ordering" link. It's been there since version 2.7.0... almost two years now

Unless you've modified the files or used a template override that affects the backend event listing, it will be there.