The manufacturer of Duratec recommends a 50 / 50 mixture of Duratec and Gel coat, and only on the finish coat unless you want to sand between coats
Remember, when you spray successive coats, let a coat cure to a tacky texture before you apply more, otherwise, you'll have too much wet gel on your hull and it will run. In my limited experience, I've found that more thin coats are better than fewer thick coats. Be patient and don't try to get the job done too quickly. I think if you went straight from one 20 oz cup to another, you'd likely end up making a mess, too much wet gel at once. I'd go with one 20 oz batch at a time. Of course the downside is that applying more thin coats and letting them cure to a tacky finish means that you'll have to clean your gun more often, but that's much better than having to sand the runs out of hardened gel-coat and filling in the thin spots. If you allow 30-40 minutes between coats for the gel to get tacky, you'll have enough of a gap to clean the gun and mix the next batch during the interim period.
What do the rest of you guys think?
I agree with M Bounds that 60-80 psi seems like a lot of pressure for an HVLP (High Volume Low Pressure) system. On the other hand, I don't feel like I ever got the optimal combination of pressure and nozzle size right, so if it's working for you, go for it. What size nozzle are you using? I look forward to hearing what you ultimately decide to do and how it turns out.
Good luck. BTW, Tennessee orange would have been a better color.