It depends. Do you want to go to the bottom of the little particles? Or just the big one. If you're interested in just the big one, use bwareafilt(binaryImage, 1) to extract just the biggest blob. If you want to get a smooth envelope that sort of goes between the large and small blobs, use activecontour (demo attached). If you want to just go to the bottom of any blob, then just process columns one at a time
[rows, columns] = size(binaryImage);
bottomRows = zeros(1, columns);
for col = 1 : columns
thisColumn = binaryImage(:, col);
botCol = find(thisColumn, 1, 'last');
if ~isempty(botCol)
% There is at least one white pixel in this column.
bottomRows(col) = botCol;
end
end
% Now find mean
theSum = sum(bottomRows);
numCols = sum(bottomRows ~= 0);
meanBottomRow = theSum / numCols;
