how can I find the cell number for the median of a specified range in a large set of data?

I have a binary picture and I have used IMPROFILE on a line across the picture, plotting 0's and 1's. then i have a 1000x1 matrix [1 column] of mainly 0's and a three groups of 1's

how can I identify for median or mean cell reference (Ie know which pixel is in the middle of a wide binary line) between 200 and 400 in my 1000x1 matrix.

---- I suppose another way would be to identify the centre of each flat peak in the improfile plot.

Any advice would be most appreciated. Thanks David