Need help with the subtle 'y' vs 'u' sound before my first coffee with a local.

I've been recording myself on my phone for weeks, but I still can't hear the difference in my vowels when I say 'tuli' vs 'tyyli'. I have my first face-to-face conversation with a native speaker coming up in Helsinki next month and I don't want to sound completely unintelligible. Does anyone have tips for mouth positioning, or should I use Chickytutor.com to get some real-time feedback on my pronunciation before the meeting? I am asking specifically about learning Finnish, not a generic study routine.