Custom SQL query: arithmetic operation fails when using parameter

Hello,
when passing a number value to a custom SQL query to make a simple division, nothing is returned. If I hardcode the number value, the query runs fine. Can anyone spot what I’m doing wrong in this excerpt, where :P1 is the parameter name:

SELECT
ROUND(SUM(tl_time_spent)*100/:P1, 0) AS 'Global_Utilization',