If within:

1-2 yrs - 10%
3-5 yrs - 5%

Gross gain is Purchase Price (when you bought) less Selling Price (current selling price). However, what is calculated is nett gain.

Thus, it is 10% or 5% of your nett gain, not from the price of the property at either time.

So, whatever costs/expenses which you spent on the property, like legal fees , renovation and upkeep costs, agent fees, can be deducted.

You can download a copy of Form CKHT 1A from the net to see what the allowable deductions are.

There's a detailed sample and calculation in v1 of this thread which may be useful to you.

Hire purchase? You mean Sale & Purchase ? Yes it is from the date of the S&P agreement.