Be aware that flooded ballast tanks reduce your speed a lot.
Don't know how much speed the negative ballast tank take from you. The main ballast tanks slow you down at surface.
Been playing with people that seems to know what they were doing and those Captains always used the negative ballast tank empty, and only flooded it when in crash dive (by the way, the negative ballast tank get flooded VERY quickly. Not like their big brothers
)