If your current feed is only 16-18%, the runny stools might not be from high protein...I thought your original post was indicating you were feeding a higher protein than that. If that is what you are feeding, it's not likely they are getting scoured from it...but more likely because of the time of year and high water consumption. You can try to bulk up your fiber by feeding a high fiber, lower protein grain like the barley to see if it will help with it, but it may just be because of the hot temps.
I wouldn't get too bogged down into protein percentages of grains...usually the cheaper the grain, the lower the protein, so just buy some cheaper grain and throw it in there at 30-50% ratio and see what comes out. I wouldn't overthink the process too much...I never do and I never calculate the percentages when I add whole grains...I just know the ones I add are less protein and so they will automatically bring the total percentage down. I don't really need to know how MUCH it brings it down as long as I keep 50% of the ration with 16% layer, it all seems to work out fine.
I know...it's just all so imprecise, isn't it?

I never really measure anything and I don't sit and calculate feed percentages...I just throw it together and watch the birds. That's all you can really do, in the end, is experiment on your own and see what happens when you do...birds can be very forgiving and you can always tweak up or down to correct any poor outcomes if you are really watching your flock every day.