Is that really true? Maybe it takes 30 min to calculate from first principles, but I'd expect you could have a book of tables that tells you with just one or two lookups.
From first principles would take a lot longer. 30 minutes is roughly what I remember to reduce about 4 stars. I could be off a bit since it's been a really long time and I forget all the steps, but I remember a lot of table lookups and interpolations to get to a final fix.
