Problem With Setting Variable Values In A Loop

In a stored procedure that I'm fixing, there is a problem with assigning variable values inside a loop. The proc is using dynamic SQL and if statements to build all these statements, but I'm having to add a new variable value to it that is throwing it out of whack.

This is the current structure:

SET @MktNbr = 10

WHILE @MktNbr < 90

BEGIN

DECLARE@sqlstmt varchar(1000)

SET @Market = '0' + CONVERT(char(2),@MktNbr)

SET @sqlstmt = 'SELECT (columns)
INTO dbo.table' + @Market + '
FROM #table
WHERE marketcode = ''' + @Market + '''
IF @MktNbr = 50
BEGIN
SET @MktNbr = 51
END
ELSE
IF @MktNbr = 51
BEGIN
SET @MktNbr = 52
END
ELSE
IF @MktNbr = 52
BEGIN
SET @MktNbr = 55
END
ELSE
IF @MktNbr = 55
BEGIN
SET @MktNbr = 60
END
ELSE
BEGIN
SET @MktNbr = @MktNbr + 10
END
EXEC (@sqlstmt)


END


I'm probably having a blonde moment, but I'm trying to replace the if statements with this:

SET @MktNbr =
CASE
WHEN @MktNbr = 10 THEN 20
WHEN @MktNbr = 20 THEN 30
WHEN @MktNbr = 30 THEN 40
WHEN @MktNbr = 40 THEN 50
WHEN @MktNbr = 50 THEN 51
WHEN @MktNbr = 51 THEN 52
WHEN @MktNbr = 52 THEN 55
WHEN @MktNbr = 55 THEN 60
WHEN @MktNbr = 60 THEN 70
WHEN @MktNbr = 70 THEN 80
WHEN @MktNbr = 80 THEN 81
ELSE @MktNbr END

Clearly it's wrong because the proc bombs every time with a duplicate table error.

It has been suggested to me that I should hold these market values in an external table. This sounds reasonable but I'm ashamed to admit that I don't know how I'd implement that. Can someone maybe give me a nudge in the right direction?