What you have done makes sense to me.
![a SU b] = Aexists({q},!q, q<->b|a&next(q),G true)
We have two states {} and {q}, both are accepting (safe) states, and {} is the single initial state. We then have to determinize it with the subset construction so that we can then negate it again to obtain a DetF automaton for [a SU b].