I would even go 20% straight in, it should cause no harm. My tap water reads at 7/ 7.1 and my tank due to CO2 is at 6.4 sometimes even 6.3 and I do 40-50% changes with no problems.
If you are going to let it sit then use a cooler and a heater for your water change amounts. I am more leary of drastic temperature changes then my ph issue.
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