Using an envelope or a mask to start phasing even with high NCS is still worse than using a low res model from cryoEM that is characterized by an (almost) proper density distribution.
One has to measure the intensities of the very low res reflections (0 0 1 and the likes) precisely.
The presence of high NCS does not guarantee a success even if your initial model is "good" as described in Section 9 here:
http://www.ncbi.nlm.nih.gov/pubmed/11526317
Petr
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Petr Leiman
EPFL
IPSB-LBBS
BSP-415
CH-1015 Lausanne
Suisse
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