Complicated Journey to enjoy the 72-hour free transit

Hello,

I have a flight ticket from Tokyo to Beijing and then from Beijing to Seoul. I'm staying less than 72 hours in Beijing so technically I could ask for the 72h transit visa. My only issue is that my first flight transits through Seoul on my way to Beijing (for 2h, so I won't even go through customs).

So my question is what matters in this case? Where I started my journey (Tokyo) or my last stop before landing in Beijing (Seoul).

Thanks a lot.