ooh..... I see... not easy to calculate it seems...
I will try reading again to see how to calcualte.
let me just give you a good recommendation video by BTCSessions:
https://www.youtube.com/watch?v=LxTkLwpV1Po&ab_channel=BTCSessionsYou can navigate in the video and find where he calculates the checksum (it's around 18min 40sec)