I use ultramarine blue as well and I use it by weight. It's amazing what a different 1/10th of an ounce will make. I use .5 ounce for a VERY vibrant blue and .3 oz for a medium blue and I mix it in with my lye water for a uniform color. If I want to swirl it, I hold back a little of my oil and mix it with that first, then put it in with a little of the soap batter, THEN I swirl. I have gotten some great results.
Also, sometimes the ultramarine can give you a little bit of an abrasive quality, so be aware of that. It's not every time and I haven't figured out the deciding factor as to why it does that yet, but it does. It's not a bad thing at all, but it is there.
HTH!!!
Good Luck!