Such good questions. I'm still kind of ignorant about these things. One of the reasons I'm opting to use both indigo and
amla w/the henna is because both of these darken the hair so that it'll hopefully turn a deep red, like what it sounds like you're aiming for.
If you want, you could wait to see my results before going totally black (although black is a pretty bad azz option)? I'm really not even sure this concoction will work. We'll see. I'll likely use 3 parts henna to 1 part indigo and 1 part amla respectively. It's going to be a minute before I actually do my head though because I definitely am taking my time to strand test until I see the exact or near-exact result I want! Also, I know my results won't necessarily determine yours because we're starting out w/different hair colors, but I could certainly share the ratios I end up using if they might help you
ETA: It just occurred to me that you don't have to wait for me to do your own tests if you already have these 3 powders. You could do your own strand/hair ball tests to see what ratios might work well for you