A Level Chem Combustion Question Help

Here is the question,
A sample of P (C6H1O2) was mixed with an excess of oxygen and the mixture ignited. After cooling to the original temperature, the total volume of gas remaining was 335 cm3. When this gas mixture was passed through aqueous sodium hydroxide, the carbon dioxide reacted and the volume of gas decreased to 155 cm3. Both gas volumes were measured at 25 °C and 105 kPa. Write an equation for the combustion of P in an excess of oxygen and calculate the mass, in mg, of P used. The gas constant R = 8.31 J K−1 mol−1.
I already wrote the equation. I did this calculation and then got an answer which was 2 mg more than what the mark scheme got. I used a different method to the mark scheme:
Mark scheme method - They found the volume of CO2 produced, used pV = nRT to find the moles of carbon dioxide produced and then used this to find the mass of P produced
My method - I used the total volume in the pV = nRT, worked out the molar ratio of CO2 and O2 from the balanced chemical equation, and then used this to find the mole of carbon dioxide produced, and then used this to find the mass of P produced.
Is my method wrong? I have used it before and it has worked out fine but I'm not sure why it does not work in this case.
Any help would be appreciated!

I don’t really see any benefit to calculating the moles of oxygen used (or a ratio thereof) or even writing out the balanced equation. They’ve implicitly told you the volume of CO2 made (it should be 180 cm^3, since CO2 dissolves in NaOH, but leftover O2 does not).
If one mole of a substance with x carbon atoms per molecule is completely combusted, you make x moles of CO2. So by using pV = nRT where p = 105 kPa, V = 0.18 dm^3 and T = 298 K, you should get n = 0.00763… mol and thus the moles of P combusted will be 0.00127… mol. From here, the rest of the calculation should be quite straightforward.
I’m not sure your method works to be honest. The total volume of gas is the combined volume of CO2 made and the remaining volume of oxygen left. There is nothing to say the ratio of oxygen left to the volume of CO2 made is the ratio as per the balanced equation - remember the oxygen was in excess but they haven’t told you how much there was to begin with. I’m surprised this method has worked before, though I’d need to see the question in order to tell you if it is complete coincidence or not.

Thank you. Yes I don't think my method worked because oxygen was in excess. I wrote out the balanced chemical equation because I had to as part of the question. I have an example question of the method I used, but I guess maybe it worked here because we know the volume occupied by the two gases, excluding any chlorine which could be in excess.
Boron trichloride (BCl3) may be prepared as shown in the equation below.
B2O3(s) + 3C(s) + 3Cl2(g) → 2BCl3(g) + 3CO(g)
A sample of boron oxide (B2O3) was reacted completely with carbon and chlorine. The two gases produced occupied a total volume of 1250 cm3 at a pressure of 110 kPa and a temperature of 375 K. Calculate the mass of boron oxide that reacted. Give your answer to 3 significant figures. The gas constant R = 8.31 J K−1 mol−1

In the question you gave as an example, it gives you the combined volumes of the two products, ignoring the leftover chlorine altogether. Because both BCl3 and CO are products (and you aren’t working with one reactant and one product), they are guaranteed to be formed in the ratio given in the equation. As such, using the ratio from the equation is appropriate.