Insofar as your questions on exposure, development times, and developer for producing interpositives with this material...that will obviously depend on the factors of your original camera negative density, your light source, and ultimately the process for which you are producing enlarged negatives.
Typically, on an contact printed interpositive, one wants a fairly low contrast. I would strive to obtain full shadow and highlight information on the interpositive with a DR of .80-.90 (high value densities minus low value densities). The next step in which the interpositive is enlarged to a new negative is where one typically builds contrast.
I have used Freestyle and Photowarehouse ortho lith film and the ISO of these materials is 6 which in the case of the Bergger film would involve a 1 1/2 stop increase in speed. When I contact print a 4X5 negative onto the films that I mentioned, with a camera negative having a DR of 1.00 ((high value densities minus low value densities), my exposures will typically run on the order of 20-25 seconds at F16. The light source that I use is my Saunders 4550 VCCE (250 watt light source) enlarger with my enlarger head height at a 8X10 inch enlargement.
If I were you I and I was working with the same light source I would begin exposing the Bergger film at 40 seconds at F32. After making one exposure you will be able to determine what adjustments may be in order. What one looks for in the interpositive is a fairly low density and low contrast with detail that correspond to the camera negative highlights. Dektol will work as a developer but it should be diluted beyond paper developing strength. I would plan on working with a 1-5 dilution (70 degrees) and plan on developing times in the range of 1 1/2 to 3 1/2 minutes. Since this is an ortho film you will be able to use your paper safelight unless it is too near the developing tray. You will be able to develop by inspection.
The desired interpositive will have detail in both the highlight and shadow regions and appear to be lower in contrast then a camera negative. Obviously the interpositive must be free of pinholes etc. that would be transmitted to the enlarged negative. From this interpositive, typically one then enlarges the interpositive onto the appropriate film for an enlarged negative. I have used the Freestyle and Photowarehouse ortho lith film for enlarged negative film. It is inexpensive and once again Dektol can be used at appropriate dilution. The desired density range for an enlarged negative will depend on the sensitometric characteristics of the process in which you engage. For platinum that would obviously be higher then the camera negative from which the enlargement originated. Some of the pt-pd photographers will be able to help you there. Good luck.
