Amount of flow depends heavily on what types of fish you plan on keeping and how much rockwork you are going to have.
Lagoon species like low flow... Open water and near surface fish love tons of movement blowing them around. But also need sheltered areas to escape the flow to rest.
Tons of rocks will need lots of flow to prevent build up of detritus, but will still create sheltered low flow areas and homes for cave dwellers.
Think about the central fish you want and do your best to recreate their native habitat. Then populate with fish from a similar area of the reef. Many species are wide ranging and very adaptable but some have very specific ranges and needs.