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That is only a reasonable approximation if you are considering a very early stage in the reaction.
Since this is the part of the reaction you are most interested in, introducing errors here would be stupid!
You have to find a way of adding the catalyst to the hydrogen peroxide solution without changing the volume of gas collected.
A measure of the rate of the reaction at any point is found by measuring the slope of the graph. Since we are interested in the initial rate, we would need the slope at the very beginning.
If you then look at the second graph, enlarging the very beginning of the first curve, you will see that it is approximately a straight line at that point.
If you were looking at the effect of the concentration of hydrogen peroxide on the rate, then you would have to change its concentration, but keep everything else constant.
The temperature would have to be kept constant, so would the total volume of the solution and the mass of manganese(IV) oxide.
The maths of this might not be familiar to you, but you may find that you are asked to do this as a part of a practical exam or practical exercise.
If it is an exam, you would probably be given help as to how to go about it.
You could also use a special flask with a divided bottom, with the catalyst in one side, and the hydrogen peroxide solution in the other. If you use a 10 cm measuring cylinder, initially full of water, you can reasonably accurately record the time taken to collect a small fixed volume of gas.
You could, of course, use a small gas syringe instead.