Not wanting to nitpick at G CEXO's answer, because its perfectly correct, I think he meant flying an "Even" flight level when he said "positive" flight level.
Yes, I actually meant even FL.
Whilst the correct answer is arrived at, I would dispute a couple of the statements in G-CEXO's explanation.
It's been a while since learning all this crap. Yes you're right, the Transition layer is actually 210 feet.