It sounds like you are having at least two problems with your mix...
First, the dose rate is 9 oz. per 100 lbs of cement, so if you have a 4000 psi mix, you likely have about 600 lbs of cement per cy, so you probably have about 50 to 60 oz/cy of Glenium 7500. Your dose rate is about right for most applications; however, it appears your dwell time is being affected by the haul time from the readymix plant. Since all of your other units are in US conventional units, I will assume that the "41 deg" is Fahrenheit. If that's the case, you should not be getting set times so quickly as you've described.
You mentioned that when you add water, it "dries up fast". This indicates that perhaps your aggregates at the plant are not being kept in a saturated, surface dry condition and they are not properly compensating with water addition at the time of batching.
I would suggest a ready mix plant audit to see if they are batching correctly.
