Thanks for that research. Interesting that diluting 50% container DEF with 50% cooler water (not distilled/deionized water) only drops the concentration by 10%.... or about 1/3. Cooler water might have a slitghtly higher density than distilled or deionized water, but won't be hugely different. Also, the % urea listed by spec on the adblue/peak bottles is measured by mass (not concentration measured by volume).
And since pure urea is more dense than pure water, that's why it doesn't go below 20% concentration on the refractometer.