I must admit I rarely measure exactly how I mix stuff ... so I can't say. I usually mix VERY small amounts, to prevent that I ruin too much ink. I think a Dinky Dip holds maybe two ml, and I'd add 3 to 5 drops of gum Arabic? In your case, I'd fill some of the ink you already mixed in a smaller container and kept adding drops.
I know how frustrating it is to buy an ink and then it's dissapointing. I bought for example Sennelier inks – it also says they are meant for calligraphy – and it's just impossible to get hairlines with it, plus it just "falls" from the nib with bigger swell strokes. A pity, they have so beautiful colors …
If it doesn't work at all to your satisfaction, you still can use the inks to make pretty background washes!
But of course, I hope it will write nicely! It helps to use really fine nibs with this kind of ink ...