Insert Multiple Rows With A Trigger That Invoke A Function

Multiple rows to insert:
---------------------
insert into Customer(CustomerId,Name,Value)
select CustomerId,Name,Value
from CustomerTemp

Trigger in Customer table that invoke a function:
alter TRIGGER [dbo].[Calculation] ON [dbo].[Customer]
AFTER INSERT
AS

update Customer
set Percentage = dbo.GetPercentage((select Value from inserted))
where CustomerId = (select CustomerId from inserted)

I'm getting the error for the multiple row.Subquery returned more than 1 value. This is not permitted when the subquery follows =, !=, <, <= , >, >= or when the subquery is used as an expression.Is there a way to let me insert multiple rows, using the trigger that invoke a function ?