50 mg is usual starting dosage, so I would try it for a day and see if you tolerate it well. If so, 50 will take effect sooner. If you have side effects that are intolerable, then cut in half then do 25 for a few days (exact number of days is not important. I suggested 25 because you are sensitive to side effects, but, pardon the complexity here, but sometimes starting too low is as bad as too much and 50 will get you to desired dose sooner). You should stay on 50mg until you are comfortable (10 days, even if you have no side effects) then go to 75 mg. You should check back with MD at that point, but usual end dosage is between 75-150mg. As we age, you metabolize drugs differently and may respond well to 75mg. You are at the age that we just have to see.
Don't worry too much about taking an extra half of Ativan. This is intended to be short term and you will likely lower dose or be able to just use occasionally once Zoloft works.
When Zoloft works you should essentially feel like you did
when Paxil worked (if I remember, it did). You should have a sense of well being and less anxious. It is hard to describe but people typically know it when they get there.
Let me know if you have any questions or issues with it while in AZ. Enjoy the trip.
I think I'll go watch the world series.
P.S. Pristiq is a good drug if Zoloft doesn't work out.