battery use

I think my battery has problem

I bought the Iphone8 last Sunday.

Yesterday, after full charged, i used my phone for 3H 53m but i got 40% left.

So i lookd up the internet about this, many people recommanded i need to use up my battery and recharge fully and i did.

But now I ve been using my phone for 2h 20m right now and i have 76% battery left.

There IS something wrong right?

Right now I am having business trip in Korea.

What should i do?