Function observed_build_time_multiplier 

pub(crate) fn observed_build_time_multiplier(
    total_work: Duration,
    work_at_tx_cutoff: Duration,
) -> Option<u64>

Computes the observed total-work to tx-cutoff-work multiplier.

work_at_tx_cutoff is measured when pool transaction execution stops. total_work is measured after finalization finishes. Their ratio captures builder_finish without needing a separate fixed reserve.