This should work for you:

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-- create a table to store our results

-- create a dummy unique identifier on the table

CREATE TABLE #Counter2

(

   RowNumber INT IDENTITY(1,1)

   ,StartDate DATETIME

   ,[Value] FLOAT

   ,Code CHAR(1)

   ,Counter INT

)

--insert all existing data from original table into our results table

INSERT INTO #Counter2(StartDate, [Value], Code)

SELECT StartDate

   , [Value]

   , Code

FROM #Counter

-- create a counter variable

-- this will be used to count our rows

DECLARE @Counter INT

SET @Counter = 0

-- this is a dummy variable

-- we need it as an anchor to keep our running total

DECLARE @RowNumber INT

-- now we will set the counter

-- we need to start the counter when the value is between 10 and 15

-- accumulate the counter until we get to 10

-- start again when we get to the next value between 10 and 15

UPDATE #Counter2

SET @Counter = Counter = CASE WHEN (@Counter >= 1 AND @Counter < 10) OR ([Value] >= 10 AND [Value] <=15)

                           THEN @Counter + 1

                           ELSE 0 -- restart the counter

                           END

   ,@RowNumber = RowNumber

FROM #Counter2

-- take a look at our results

SELECT *

FROM #Counter2

-- clean up

DROP TABLE #Counter2

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