Once you find a placement that works for you, you can lock it using the Design Wide Resources > Directives.
Add a directive and choose Force Component UDB.
The signal names and UDB location are in the RPT file. In the Workspace Explorer > Results Tab.
In the RPT file, search for the name of the UDB component you created.
For example, a control register (Control_Reg_1) might show like this:
UDB [UDB=(1,5)] contents:
controlcell: Name =\Control_Reg_1:Sync:ctrl_reg\
If that's the case, add a directive like this:
Component (Signal) Name: \Control_Reg_1:Sync:ctrl_reg\
Directive Type: ForceComponentUDB
Directive Value: U(1,5)