From my limited work on vintage LF SLR's a improperly positioned view screen/view ground glass will make a square or rectangle appear as a trapezoid if the alignment is incorrect.

Any focused distance can be used for alignment as the optical distance to the image surface of the view screen/ view gg must be equal to the optical distance to the film plane. As I stated in a response to one of your other threads focus at the film plane then adjust the focus screen/view opening ground glass to match the film plane.

A ISO 1223 test target, http://www.graphics.cornell.edu/~wes...res-chart.html , or a USAF 1951 lens test target, http://en.wikipedia.org/wiki/1951_US...ion_test_chart can be used for alignment purposes. Print out a high resolution copy on paper or transparency film on you photo printer.