As I mentioned above the x-ray duplicating film is like making a paper print. Obviously the better the negative you start with the less you might have to burn and/or dodge. Most of my exposures are also in the 3 minute range (depending on the density of the original). Make a test as you would a test print. The GBX chemistry is available from dental supply houses and is good for about a month once diluted to make the working solution. A few years ago I enlarged and printed a limited edition - 12 numbered prints each of 4 images for the photographer Mario Algaze from his 2 1/4x2 1/4 negatives. He is an internationally known and collected photographer. It was quite an experience but the results were exquisite.
I don't do photography for a living and doubt that I will print for someone else again ... who needs that stress. I'd rather just have to please myself.