default
The fraction used when none is given.
Zero, so that constructing this class without a deliberate choice cannot silently change what the method does.
Where a fraction is wanted, the pilot puts it near one fifth. That is a compromise rather than an optimum: one tenth minimises the distance to the truth at poor separation but is the worst fraction for recovering the number of components, and one fifth is the only value that is good on distance, coverage and count at once. Which of those matters is a question for the caller, not for a default.