TurboTax uses the declining balance method of calculating CCA because the CRA requires most classes to be calculated this way. Capital cost allowance on a few properties, such as leasehold interests (class 13), and patents and franchises (class 14) may be calculated on a straight line basis.
If your CCA should be calculated on a straight line basis, manually calculate the CCA, and then change the amount at the end of the CCA section of the interview.
To calculate the CCA manually, get the self-employment form that applies to you from the CRA. Manually calculate the CCA for the entire class on the CRA form.
Repeat the process for each class in which you have property that is calculated on a straight line basis.
You can find most CRA forms and publications on the CRA's Forms and Publications web page.