Getting Last (newest) One Record (datetime Column Or Id)

Hello everybody,

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CREATE TABLE [T1] (
[IDX] [numeric](18, 0) IDENTITY (1, 1) NOT NULL ,
[DateEvt] [datetime] NOT NULL,
[Value] [varchar] (10) NOT NULL ,
[DataX] [varchar] (10) NULL ,
CONSTRAINT [PK_T1] PRIMARY KEY CLUSTERED
(
[IDX]
) WITH FILLFACTOR = 90 ON [PRIMARY]
) ON [PRIMARY]
GO


insert into T1 (DateEvt,Value, DataX) values('2004.10.10 10:00:00',
'0000000001', 'AAAAAAAAAA')
insert into T1 (DateEvt,Value, DataX) values('2004.10.10 10:00:01',
'0000000002', 'AAAAAAAAAA')
insert into T1 (DateEvt,Value, DataX) values('2004.10.10 10:00:02',
'0000000003', 'AAAAAAAAAA')
insert into T1 (DateEvt,Value, DataX) values('2004.10.10 10:01:00',
'0000000001', 'BBBBBBBBBB')
insert into T1 (DateEvt,Value, DataX) values('2004.10.10 10:02:00',
'0000000001', 'CCCCCCCCCC')
insert into T1 (DateEvt,Value, DataX) values('2004.10.10 10:03:00',
'0000000001', 'DDDDDDDDDD')
GO

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and the question is:
In which fastes and best for the preformance way, get the last IDX of
specified Value.

I could do this like this:

-------------------------------------------------
declare @nIDX numeric
declare @sValue varchar(10)

select top 1 @nIDX = IDX from T1
where Value = @sValue
order by DateEVT desc
-------------------------------------------------

But I know, this is not fast (even if I have index on DateEVT field),
and I'm quite sure, that there is better way to get this IDX.
Anyway, this table can be big (like 20 milions records).

I could take the max of IDX, but is it a sure way?
Any help? Thanks in advance

Matik