Yes the Valmet procedure is a special one. I know one papermill that have their own roll shop and use eddy probe for the middle plane.
Valmet have procedure where a stiff machine on stiff foundation is mandatory, if I remember correctly, it was not working on a clay ground in factory in Sweden only on bedrock in Finland.
They make a run at the bending mode for the middle plane to correct for that. Was it run at half critical speed or something similar special? Long time ago.
When on rare cases we balance rolls we also only use 2 plane, it is tricky to reach the middle plane in the machine on site unless it has a special mechanics for that. Guide rolls used to be run in the lathe where it was made, balanced in 2-plane. Since rolls, cylinders are so slinky we try to run at the speed the machine normally run at.
In this case the machine actually has a third bearing mount with sensor for the middle support bearing on a drive shaft.