Phil -- he isn't saying that the computer is used 40% of the time for keeping track of the rental property; he's saying by square footage the rental property is 40% of his total property -- so he's taking 40% of utilities etc. as deductions against rental income, and he's concerned that he can only depreciate 40% of the computer cost.
CRA will say that you can depreciate, using the appropriate CCA rates, a personal asset that you also use for business (i.e., a car, a computer) in accordance with the proportion of business use you derive from the asset compared to personal use. If you get some personal benefit from your computer, you cannot (or at least should not) write off the entire cost.
(Also, the OP seems to be mixing two issues together -- the rate of depreciation, which is established by CRA for depreciable assets; and the proportion of use of a personally-owned asset which is attributable to the business. If you own an asset personally, and intend to write off 100% of the costs of that personally-owned asset, you will still need to take CCA on the asset over time. You will not be able to take the full cost of the asset as a deduction in the current year.)
With a car, there are straightforward ways to calculate the proportion of personal benefit and business use -- using the "proportion of KMs driven for business and total number of KMs driven in the year" calculation. With an asset like a computer, the question is less straightforward.
I don't give advice over the Internet. Hence my recommendation for the OP to meet with a tax professional and get answers that way.