I am using the standard Linhof ground-glass. no center filter and f5.6 viewing aperture. as I recall, the consensus was not all that positive about the gains from other viewing screens related to the costs. it sounds like a fresnel, much like what is included with my Hasselblad Flexbody for tilt viewing, would be valuable.
while I appreciate the Polaroid recommendation, the environment disallowed that luxury. lighting and subject transience presented overwhelming difficulties coupled with difficult focusing. there is good central focusing, but I am unable to see any image at the corners or enough to evaluate movements.
I have a project needing the 58XL, which I now hestitate to purchase, after experiencing these wide-angle difficulties. it might be easier though, since movements are unavailable.