Just wrap ABS around CALCULATE
Net Timing =
ABS(
CALCULATE( SUM( metricssalescustomer[amount] ),
metricssalescustomer[sales_type_lvl1] = "Lost - Timing"
|| metricssalescustomer[sales_type_lvl1] = "Decline - Timing"
|| metricssalescustomer[sales_type_lvl1] = "Upsell - Timing"
|| metricssalescustomer[sales_type_lvl1] = "New - Timing"
)
)
I hope this is helpfuf