[phenixbb] ENSEmble parameters when the model is an EM map

Dear benevolent experts, I am trying to use Phaser for molecular replacement with EM-map as a model. The EM map, originally in a small P1 cell, was put in large P1 using CCP4's MAPROT and a mask that was defined by a certain cut-off level. Could you please advise me how to derive the EXTEnt and CENTre parameters needed for ENSEmble building ? Does the simple way of giving the size of the original P1 as the EXTEnt and its centre as CENTre sound reasonable ? I would be grateful for any advice and comments, Peter.