Trial and error really is the only way. A good bike shop will let you return a saddle within 30 days or so assuming it is undamaged. Some manufacturers also have actual test saddles (fizik, specialized, wtb) that you can borrow from shops for free.

The specialized meter is sort of useless IMO. I went to one shop and a guy told me that I definitely need a 155mm saddle. The next day I visited another place and the person said 143. Since no one actually has 155mm saddles in stock, I tried out a 143 mm Toupe and it seemed to work fantastically (aside from my sit bones being sore after about 4 hours unless I got into the drops for a few minutes). I'm now trying out a 143 mm Romin and it seems to be a winner. I do wish that I had access to 155mm saddles but if the Romin holds up on the 80ish mile ride I am planning to take tomorrow then there's no point in seeking one out.