Absorption Spectrum Using a Spectrophotometer
This is simple mathematical conversion. Firstly, you find the fraction of each absorbance value. 

Let, the absorption spectrum has a three absorbance values say $A_1, A_2$ and $A_3$. 

Now, find the fraction of each absorbance as per following:

fraction of $A_1 = A_1/(A_1+A_2+A_3)$

fraction of $A_2 = A_2/(A_1+A_2+A_3)$

fraction of $A_3 = A_3/(A_1+A_2+A_3)$

Now convert all the respective fraction values into percentage as per:

percentage of $A_1 = (\text{fraction of }A_1 \cdot 100)\%$

percentage of $A_2 = (\text{fraction of }A_2 \cdot 100)\%$

percentage of $A_3 = (\text{fraction of }A_3 \cdot 100)\%$