[msyounas]

What is the exact procedure to calculate E Value of any OBA. details plz !

[DrCMS]

I assume you mean the E₁¹  which is the absorption of a 1% solution with a 1cm path length.  Typical pure solid OBA's have values from 400-1000.

You can not accurately measure the absorption of a 1% solution so you dilute the sample further.
Dissolve the sample enough to obtain a UV spectra for 1 cm path length cell that is on scale for the instrument you have.  Take care to keep the solution in the dark as dilute OBA's can quickly photoisomerism to optically inactive materials.
Determine the absorption at the lambda max and back calcalculate what it would be for a 1% solution.  That calculated value is the E₁¹