Fitted a Shimano XT Shadow derailleur and a new chain to my bike. I used the method of putting the chain around the largest chainring/sprocket without the derailleur then adding one link. But the chain is visibly slack in the smaller sprockets with the granny ring, and the derailleur is pretty horizontal when in the biggest sprockets!

Not sure if this is right!

that sounds about right.

I always understood it to be Big/big and remove two links, remembering to make some correction for chain growth if you have a full susser.

Big/big and add two links is the official Shimano method, have you got a short caged (GS) mech by any chance?