You’re correct that 70% isopropyl alcohol is used for these ink washes. In the April MR Cody Grivno suggests using 2 teaspoonfuls of ink (both India and Sepia) per pint of isopropyl alcohol. I usually mix mine by eye but this ration comes pretty close.
I like to use the higher % of alcohol, 90% if I can find it. It dries/evaporates just a tad faster which is one of the goals. I was able to buy, at a surplus store, a very inexpensive spray bottle, rather squat rather than tall, perhaps meant for cheap perfume? It cost 35 cents. What I like best is that it is translucent so I can easily see how much I have inside. But frankly I expect any spray bottle of the kind you might routinely throw away every week, from hair spray to cleaning products to room sprays, would work just fine. I have had no problems with storing my india ink wash solution in those bottles for years. Normal handling seems to slosh it around enough to keep it well mixed.
As to the right mix, trial and error works for me but I think a case can be made for having two bottles: a very light mix and a very dark mix for different applications and uses.
I also use one of those cheap spray bottles for pure alcohol, used to wet down ballast before cementing. It has many uses.