Definitely, under-exposed! I have used Ulano's (diazo-sensitized) RLX for years...I have had COUNTLESS sales reps foist photopolymer samples on me over the years and have NEVER got a useable screen out of it! With our exposure unit, ( A glorified tanning bed with fluoro UV bulbs) and the fact that I am stuck in "Vellumopolis" with all of it's attendant weaknesses (Lack of image density, and a non-transparent carrier) I absolutely NEED the exposure latitude that Diazo emulsion affords. I use from 60 mesh up to 230, and times range from 20 minutes at the coarse end of the spectrum to about 3 minutes at the fine end. I can not begin to speculate HOW one could realize useable screens at one exposure time...Guess I need to get out more, LOL!
Up your time, and you'll be fine!