I think youy are assuming a certain level of "quality control" and "regularity" between caps, and suppliers of caps.
There is no "standard percentage" of corn starch, as if the DNP capping process was goverment regulated, or done by Eli Lilly or whatever.
Chances are these caps are made by some dude in his basement wieghing the DNP on a scale, an then just adding corn startch to it.
Again, bro, ... you did not exactly go to your local Walgreens to buy these things - right? I think you are a bit misguided about what to expect.
If you are at all concerned about doseage, and/or if you have never done DNP, and/or if this is a new supplier, I would highly reccomend doing like a 7-10 day run at a very low dose, like 200 mgs/day.
Or, I would run 200 for like 4-5 days and see how it feels, then bump it to maybe 400 (if your pan eventually is to go to a higher dose for a later cycle) for like the last 3-4 days maybe, just to see how it goes.
This wil help you to test the dose of the caps, and test for your own personal reaction to DNP - quite imprtyant if you have never done it before.
If you know anything about DNP you know to wait DAYS before you can even start to cliam that you know how it "feels". So dont rush things, try a low dose for few days and see what happens.
I would try that route, and then if all goes well, take a week or 2 off, and the do your full run (again, that is assuming your plan is to do like a 400.mg-day cycle).
I hope the response back is not "well, I only got enough to do 400 mgs per day for one 10 day cycle", well, if you did, you lose.
Hope that helped.