Does Bangkok immigration add or deduct days when applying for a visa extension before it expires?

As a rule extensions are added to the expiration date of your current stamp

The only time an extension isn't added to your current expiration date is if you are getting an extension for a different reason than you have the current visa/extension

In Bangkok you can go 45 days before your current stamp expires to apply for an extension

What stamp are you on now and what extension are you trying to get ?