For change_in_volume_per_visit, your denominator is wrong. Your denominator should be your LAG function LAG(main_total_size, 1) OVER ( PARTITION BY linked_lylty_card_nbr ORDER BY start_txn_date)
LAG(main_total_size, 1) OVER ( PARTITION BY linked_lylty_card_nbr ORDER BY start_txn_date