This is what I understand about the proj_thresh threshold (please, someone correct me if I'm wrong): If you're generating 5000 samples from each voxel (the default) and guess you want to calculate the proportion of samples that arrived to a certain target only for those voxels from which more than 25% of samples arrived, then the threshold input in the proj_thresh command would be 1250. If you want to calculate for all voxels, no matter the percentage that arrived, then would be 0.
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