Proration Issues with database items

Hello, I am having some issues with Oracle Fusion Cloud. I am new to this platform from R12.

Now I am trying to do a basic proration.

I have written my proration formula which works:

/* VARIABLES */
DEFAULT FOR PRORATE_START IS '01-JAN-1980' (DATE)
DEFAULT FOR PRORATE_END IS '31-DEC-4712' (DATE)
DEFAULT FOR PAY_EARN_PERIOD_START IS '01-JAN-1980' (DATE)
DEFAULT FOR PAY_EARN_PERIOD_END IS '31-DEC-4712' (DATE)
DEFAULT FOR PAY_VALUE IS 0


/* INPUTS */
INPUTS ARE PAY_VALUE, PRORATE_START (DATE), PRORATE_END (DATE)

/* CALCUATIONS */
TOTAL_DAYS = DAYS_BETWEEN(PAY_EARN_PERIOD_END, PAY_EARN_PERIOD_START) + 1
WORKED_DAYS = DAYS_BETWEEN(PRORATE_END, PRORATE_START) + 1

PAY_VALUE = ROUND( PAY_VALUE * (WORKED_DAYS/TOTAL_DAYS), 2)

/* RETURN */
RETURN PAY_VALUE

Now I have attached this proration formula to an element, Basic Pay Element.
THe formula for the element is like this:

DEFAULT FOR TEST_ELEMENT_AMOUNT_ASG_ENTRY_VALUE IS 0
DEFAULT FOR PAY_VALUE IS 0
DEFAULT FOR MONTHLY_GROSS IS 0


/* INPUTS */
INPUTS ARE MONTHLY_GROSS, PAY_VALUE

/* CALCUATIONS */
PAY_VALUE = TEST_ELEMENT_AMOUNT_ASG_ENTRY_VALUE * 0.25

MONTHLY_GROSS = TEST_ELEMENT_AMOUNT_ASG_ENTRY_VALUE

/* RETURN */
RETURN PAY_VALUE, MONTHLY_GROSS

Now this calculation uses the Amount input value of an element called Test Element. So to get the value I am using the DBI, TEST_ELEMENT_AMOUNT_ASG_ENTRY_VALUE.
However, for the current month, I have updated the value of Test Element in the middle of the month.
This has triggered proration so I get two results. But the problem is that during the calculation, the value of the DBI TEST_ELEMENT_AMOUNT_ASG_ENTRY_VALUE is the same. That is the value it is using to calculate PAY_VALUE and MONTHLY_GROSS always uses the latest value of TEST_ELEMENT_AMOUNT_ASG_ENTRY_VALUE instead of the value as at the proration period.

Any ideas?