There is no golden rule. We all know that by now. If you dedicate an hour out of every day to stretching and core exercises you will notice you can tolerate more and more drop. It's a function of of how flexible you are. If you can ride with your neck cranked back and have lots of flexibility in the lower back and pelvis you can handle lots of drop.

You can tell how flexible someone is by how they set up their bike. If they have a huge drop with their seat tilted forward you know they are not flexible because they are not able to rotate their pelvis back to take the pressure off the soft tissues, hence the tilt. You will also notice that many people with tons of drop ride on the tops 95 percent of the time... it would be way to uncomfortable to ride in the drops... or they ride with their hands right by the stem... inflexible... It's fine to have huge drop on a track bike, unless you are a pro you never spend more than 15 minutes on the bike... unless you are making an hour attempt... My track bike would be uncomfortable for anything other than the track!

My road bikes have at most 3 inches of drop and yes I ride in the drops at least 50 percent of the time. My touring bike has NO drop because I can be on it for as much as 8-10 hours in a day! My biggest drop is close to 4 inches and I wouldn't take that bike on anything longer than 2 hours. I'm not that flexible. But I am doing yoga and guess what, I'm more comfortable with drop now than I was a month ago.

I'd say go to the golden era for the rule of thumb. Most riders have nearly horizontal backs at time but their elbows are at 90 degree in the drops... can you do that? That is about the most I will go with or... if I can't use a cinelli stem and a 180mm campagnolo seatpost.. it's too small.