Assuming percents to be mass per mass, then you need these:
85% ----------- 1.689 grams/ml density
15% ----------- 1.0824 grams/ml density
w, mass of water to add
Solve this to get the MASS of water to use, in grams... and then you can convert into its volume if you wish...
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online ref, https://wissen.science-and-fun.de/chemistry/chemistry/density-tables/density-of-phosphoric-acid/