How do I determine "adjusted cost base" if I have cost or book value (box 20), proceeds of disposition (box 21) and quantity of securities (box 16) (for mutual funds)?

[Kay-Eff]

I'm assuming it's going to be a calculation of some sort. Can you give me a clear example of I give you the fake values for box 20 = 11, box 21 = 10, and box 16 = 50.85? I have taken a loss in the sale. I'm a bit frustrated that TurboTax doesn't match the layout of the T5008 and leaves me guessing.

[TonyC11]

The error is with slip issuers in that hey don’t stick to one format, nor do they calculate box 20 unless you ask them to. 

ACB box 20 is not calculable from box 21, they are separate items.

The equation to be solved is this:

(Net Proceeds of Disposition)- (ACB of assets disposed of) = Capital Gain/Loss

Net Proceeds will be box 21 less any costs incurred in selling the assets .

ACB is the original cost, with adjustments for any subsequent purchases or return of capital.

See CRA Guide to Mutual Funds forexamples of ACB calculation.