Cgt individual (wasting assets that r not chattels)

Hello sir. I didnt understand the calculation of wasting assets that are not chattels. In numerous notes i have found that farmula is
Remaing usefull life/total life x (c-s).. where as farmula in OT notes is
Life already been used/total life x (c-s)
And one other thing u said the figure that will come from the farmula will reduce the allowable cost and then this fig will be deducted from net sale proceeds to calculate gains. Where as in other notes they have directly deducted the fig came from the farmula from the sale proceeds to claculate gain. Please explain sir. Did i mix myself somewhere? I hope i have made my question clear?