You were just in time!
I was getting really geared up to give you a thorough bashing, but now that I saw you found out yourself, let's keep it to this :
Ofcourse you don't have to add another 200 ml water to the already 30% water part of the 70% IPA !
Just saturate the 30% waterpart from storebought IPA.
And warm up the IPA to f.ex. 40-50 C, then saturate with pure NaCl, so you are sure that when cooled down, the waterpart is for sure saturated 100%. You will see then salt precipitating out again, indicating a 100% saturation.
Please don't try to break through the soundbarrier next time again with a bycicle, hehhe. LT/