Just remember, all this RNG thing is pure crap. At a point, I was used to do Soul+III on a 1 slot wep/armor. That is 37% and 42% IIRC. And it used to work 1/3 of the times. Then Dodo started to hate me and fail most of 80% tries.
2 slots target lets you a lot of combinations. Even with low chances, it may work even adding a slot in the process. It's up to your fodder and mood. If you have already 1 slot, you can add directly soul. That is 40% for soul, and 70% for low level slot. In fact, unlocking a second slot, prior to add Soul, only gives you 10% higher chances on that Soul. 100% on fodder slot, that is also true, but 70% is already a good chance.
Then 2 fodder souls, and one or two III. Trying to be cheap here may cost you a lot of meseta.
If you are going to try Soul+III+Boost, save at least 1M (that you will lose), an afix booster, and do the safe way. It's true I did almost all my spirita boost adding a slot after failing the safer way, but also that I burned some Spirita Boost items, and they aren't cheap. You will notice then how expensive is to afix 10* items, just trying to set it to 3 fodder slots may eat that million meseta.
As an advice, don't try to do your III fodder yourself. Buy it between 1k and 10k or farm it at AQ's. Soul+III fodder is around 250k on ship 2.
And remember that Dodo is just like gambling, stay away from him as much as you can. An extreme poor man is talking.
