FYI, In Canada, if you are short a stock & have to pay the dividend, you CANNOT reduce the dividend paid from your adjusted cost base of the shares. It is also not a carrying cost. For tax purposes in Canada, it is technically a non event. In other words, you pay it & can get no deduction for it either as a deduction from your cost basis nor as a carrying cost. Check this out:
Here is a link to an article I came across as well as the applicable para...
Payments made to compensate a securities lender for dividends on a borrowed share of a Canadian issuer will generally not be deductible unless the securities borrower is a registered securities dealer in which case two thirds of the payment will generally be deductible