Overflow error when using the sum function? - sql

Question

I'm writing a query to calculate how much more the average customer has transacted and spent since signing up with my company. However, it returns the error message "Overflow occurred during numeric data type conversion".
The problem is definitely this bit, as when I comment it out the query runs successfully:
SUM(SALES_GROWTH_PERCENT)
Google tells me it's likely that I'm trying to fit a number with too many decimal places into a field that's too short for it. However, I've tried changing how many decimal places the case statements are using with no luck.
Edit: for context, here are the details of the fields being used, from syscat.columns:
This is the query in question. Can anyone see where I'm going wrong please? Any advice would be appreciated!
SELECT SUM(TRANS_GROWTH_PERCENT)/COUNT(CUSTOMER_ID) AS AVG_TRANS_GROWTH_PERCENT,
SUM(SALES_GROWTH_PERCENT)/COUNT(CUSTOMER_ID) AS AVG_SALES_GROWTH_PERCENT
FROM
(
SELECT CUSTOMER_ID,
CASE WHEN POST_SIGNUP_TRANS <=0
THEN 0
WHEN POST_SIGNUP_TRANS >0
THEN CAST((CAST(POST_SIGNUP_TRANS AS DECIMAL (10,5)) - CAST(PRE_SIGNUP_TRANS AS DECIMAL (10,5))) / CAST(PRE_SIGNUP_TRANS AS DECIMAL (10,5)) * 100 AS DECIMAL (10,1))
ELSE NULL
END AS TRANS_GROWTH_PERCENT,
CASE WHEN POST_SIGNUP_SALES <=0
THEN 0
WHEN POST_SIGNUP_SALES >0
THEN CAST((CAST(POST_SIGNUP_SALES AS DECIMAL (10,5)) - CAST(PRE_SIGNUP_SALES AS DECIMAL (10,5))) / CAST(PRE_SIGNUP_SALES AS DECIMAL (10,5)) * 100 AS DECIMAL (10,1))
ELSE NULL
END AS SALES_GROWTH_PERCENT
FROM
(
SELECT CUSTOMER_ID,
SUM(PRE_SIGNUP_TRANS) AS PRE_SIGNUP_TRANS, SUM(PRE_SIGNUP_SALES) AS PRE_SIGNUP_SALES,
SUM(POST_SIGNUP_TRANS) AS POST_SIGNUP_TRANS, SUM(POST_SIGNUP_SALES) AS POST_SIGNUP_SALES
FROM
(
SELECT CUSTOMER_ID,
CASE WHEN TRANSACTION_DATE >= ANALYSIS_START_DATE THEN 1 ELSE 0 END AS ANALYSIS_FLAG,
CASE WHEN TRANSACTION_DATE < SIGNUP_DATE THEN 1 ELSE 0 END AS PRE_SIGNUP_TRANS,
CASE WHEN TRANSACTION_DATE < SIGNUP_DATE THEN SALES ELSE 0 END AS PRE_SIGNUP_SALES,
CASE WHEN TRANSACTION_DATE >= SIGNUP_DATE THEN 1 ELSE 0 END AS POST_SIGNUP_TRANS,
CASE WHEN TRANSACTION_DATE >= SIGNUP_DATE THEN SALES ELSE 0 END AS POST_SIGNUP_SALES
FROM TRANSACTIONS_TABLE
)
WHERE ANALYSIS_FLAG = 1
GROUP BY CUSTOMER_ID
)
WHERE PRE_SIGNUP_TRANS >0
AND PRE_SIGNUP_SALES >0
)
;