There are some great graphs at JW facts that looks at this information, although they don't have the last few years.
They basically used the method as you did. They note that there is a time difference between when a person starts publishing and when they get baptized, but they feel the numbers average out over time. While it's not exact, I do think looking at this numbers in this way does tell you that a lot of people leave or at least stop publishing each year. The increase in publishers is only about half the number baptised, and this is quite a change from prior to 1990.
This is one of the most telling graphs, showing that the number of people leaving has tripled from the rate in the early 1990's. For the 10 years from 1986 to 1995 the rate was an average of 12%; for the 10 years from 1996 to 2005 it had risen to 41%.
So there have been clear indications that a lot people were leaving as far back as 1995. Twenty years of slower growth and a lot of churn, but it seems that it is only recently that the governing body realized they have a serious problem. They are trying to change now , but I think it's too little, too late
JW facts also look at the conversion rate, or how many get baptised compared to the number of bible studies. It's very telling, the number of studies increased, while baptisms plummeted.
Whilst there is roughly one Bible study conducted every month for each publisher, only a minute amount of these ever progress to baptism. Although the number of studies doubled in the 20 years from 1995 to 2014 to 9 million, the number of baptisms plummeted from over 350,000 per year, flatlining around 275,000.