Whether the stock bounces from the 16-month low it hit Wednesday or sinks even lower may depend less on iPhone or iPad sales than on its guidance for the June quarter (more on that later) and what it reports about a relatively obscure ratio closely watched by analysts: the company’s gross margin, or GM%.

Gross margins are calculated by dividing the difference between revenue and cost of sales by revenue (the formula is GM=(Rev-Cost)/Rev), and they are considered the best single measure of how efficiently a company like Apple turns sales into profits.

“Gross margins are likely to shape sentiment surrounding Apple’s stock,” Bernstein’s Toni Sacconaghi wrote in a note to clients Wednesday, “as they are key metric underpinning both the bull and bear cases.”

• Apple bears, according to Sacconaghi, believe that the company’s enviably high margins, which peaked at 47.4% a year ago, are unsustainable given the competition from cheaper Google (GOOG) Android devices and the pressure on Apple to respond with lower-priced (and lower margin) iPhones and iPads.
• The bulls, by contrast, believe that the unusually low GM of 38.6% that Apple reported in January was due to the unusually large number of new devices the company released in September and October last year. The company’s gross margins, they believe, should steadily improve in 2013 as the costs associated with bringing out new products come down and the economies of scale kick in.

In January, Apple management said it was confident the company could report gross margins for the March quarter somewhere between 37.5% and 38.5%. Most analysts expect Apple to come in at the high end of that range. In fact, among the 60 we polled — including 37 Wall Street professionals and 23 amateurs — the median estimate was exactly 38.5%.

Below: The individual analyst’s estimates — pros in blue, amateurs in green. We’ll find out who was closest to the mark when Apple reports its fiscal Q2 2013 earnings after the market closes on April 23.

Thanks once again to Posts at Eventide‘s Robert Paul Leitao for pulling together the Braeburn Group numbers.

NOTE: An earlier version of this story transposed two numbers in Ovi Popescu’s estimate. He’s forecasting a GM% of 39.5%.