Self-employed

The amount on my T2200 only covers 3200 kms based on the CRA rate of .54/km and is not included on my T4 slip as taxable income.  My employer pays $12.00 for every 100 kms driven daily with a 40 km/day deductible, meaning I have days where I receive no consideration for kms driven.  I was told that I could voluntarily enter the amount on my T2200 as Other Employment Income and then claim all of the kms driven for employment, however I don't know how to do this?  Do I claim the kms as an amount (for example, first 5000 kms at .54 and remaining at .48 and enter the total somewhere?  I'm a homecare PSW if that helps.