The successful thumbnails that I recall (full disclosure, no great experience here) have a longer, more gradual, transition along the length. IMHO a thumbnail is like an in-motion tuck point shrinking approach. You're trying to persuade the metal to shrink. If you have a short, rounded male, you run the risk of both stretching at the same time your shrinking and this may be the cause of your "fatigue and split" results.
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Marc
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