You're wrong. Light and player level are separate. You will be level 20 and your light level (which no longer affects player level) will be whatever gear you are wearing.
I guarantee you will need 20 levels of experience to do to reach 40. Your character level is level 20 now, and it will be 20 when you start TTK.
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https://mobile.twitter.com/urkmcgurk/status/629810426914701312. No, you're wrong, can you read man?