Bios checksum on MSI

Hi!

I have an MSI GL66 Pulse UEK, That i modded the bios for. I have done it in the past and i can do it again. But when flashing I get a weird error and after some research I think its because a checksum is not right. The bios comes with a .txt with the filename and a checksum. I need to remake that checksum but i dont know how they made it.

Thanks,

Joran

Bios: https://download.msi.com/bos_exe/nb/E1581IMS.312.zip