Quote Originally Posted by MartinP View Post
So, what we really could do with is a list of tips for using Portra (sheet, roll or 135, as appropriate) as an interneg or dupe film, in order to print from transparencies on to RA4.

Several decades ago this was a standard part of our lab work, but there was/were specialist interneg film(s) from Kodak for exactly this purpose - they were discontinued years ago of course.
Use about a 100C and 50 M in your enlarger to simulate daylight and use about a 1/2" exposure at F22. This should make a nice 3x4 onto Portra 4x5 from a 35mm.

For process, pull using a 3' development time or a 2' 45" development time to reduce contrast.