Some races enforce their cutoff on the gun clock rather than your chip time. Check what your race says, then see where you land against whichever clock it uses.
Already know your race? See what it publishes about its cutoff, sweep and walker policy.
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This is arithmetic on a constant pace. It does not account for fatigue, heat, hills, injury, or anything else that happens after mile 18. Treat the buffer as the best case, not the expected case.