Compression test on a 2006 x-type

Anyone done a compression test on their engine recently?

A friend’s appears to be losing pressure. It’s a 2006 x-type, 2 litre diesel
What numbers should i be aiming for??

Just ordered a kit online. Is the kit easily applied??
Haven’t used one before

Thanks

Looks pretty straight forward to do.
Picked one up pretty cheap online: https://www.greasemonkeydirect.com/collections/silverline-tools/products/diesel-engine-compression-testing-kit

Pressure values are as per the Jag specs for when the car rolled off the production line.
Worn o-rings and carbon deposits are the culprits for loss of engine power.