I suggest you specify the percentage of values that will become 1, and use the corresponding percentile value as the threshold (computed with prctile
function from the Statistics Toolbox):
x = [3 45 0.1 0.4 10 5 6 1.2];
p = 70; %// percent of values that should become 1
threshold = prctile(x,p);
x_quant = x>=threshold;
This approach makes the threshold automatically adapt to your values. Since your data are unbounded, using percentiles may be better than using averages, because with the average a single large value can deviate your threshold more than desired.
In the example,
x_quant =
0 1 0 0 1 0 0 0