How to find mixed layer depth using the criteria mentioned?

I am having a 3-D dataset of density.(360x180x42)(lat long depth). I want to store the depths at which my density changes by amount of 0.03 for the first time as we go from top to bottom. Problem is no exact difference is 0.03. They all are nearby 0.03. So, how to tackle this. Any ideas?

[jonas]

Depends on the characteristics of your data set. Is the density (d) change ( diff(d) ) steadily increasing with depth? Then find the first value where diff(d) exceeds 0.03.