Depreciation on a cellphone

I have a pay as you go cellphone that I bought cash. Thus I unfortunately do not have monthly statements to determine an exact break between business and personal use. I spend about R300 pm on airtime and I receive a cellphone allowance of R750 pm from my employer. How do I calculate and show the depreciation amount and do I need a letter of consent if the allowance is shown on my salary slips?

Please make use of our Wear and Tear Calculator to assist with your workings.
If SARS does request supporting documents, you may need to provide a letter from your employer and you will also need to provide some justification for how you calculated the spilt between personal and business use.