Set circumference

[Gabrial]

Hello everyone,,
I am the owner of an Arion digital B +, and I am using the My and training software, I wanted to know how I should sector the circumference, I use it with a 29 "mtb bike and with road tires, I read almost everything about how set the wheel circumference but honestly I'm a bit confused, to measure I used the technique of rolling the wheel on the floor but then I don't know what measures to insert in the special section, is there anyone who wants to help me? Thanks