An interesting question, and as pointed out by others, there are so many variables which may impact the calculation, that it is difficult to model from scratch.
One option though, is to turn to history. It is our good fortune that the British War Office published detailed records of all manner of things for the First World War. Relevant for these purposes, this included all casualties by type by month for the Army in France. Should you wish to follow up, or learn more, the work is titled Statistics of the Military Effort of the British Empire During the Great War: 1914-1920.
In formulating this response, I have restricted the analysis to the ‘killed’ category, ignoring the ‘died of wounds’ and ‘died of disease’ ones in the process. I’ve also summed across the ‘Regular Army’, ‘Territorial Army’, ‘Canadian’, and ‘Indian Native’ bandings. 342
Unfortunately, there is no detail on the ranks of the officers killed. However, the ratios of officer ranks indicated by another response here could be used to estimate that level of detail. Finally, since seasons are important (the most conspicuous day of European war history being 22 June), I’ve restricted the analysis to the month of April for each of the years. Doing so, I have found the following:
1915: 5.25% (288 officers and 5,197 other ranks)
1916: 4.43% (157 officers and 3,386 other ranks)
1917: 5.9% (1,424 officers and 22,700 other ranks)
1918: 6.58% (949 officers and 13,475 other ranks)
As an observation, there is little variance in the percentages, in spite of a steep increase in casualty totals for 1917-8. Also, although not displayed here, the ratios hold true for the individual banding, suggesting that the ratios may be a good indication of the OCP. If we bear in mind that the British Army in France for World War One effectively represented three ‘generations’ (the original, professional and experienced army – all but wiped out by 1916; Kitchener’s Pals Battalions – a tragic volunteer force; and the final conscripted army), then it is encouraging to observe that the ratio holds true in spite of levels of experience and professionalism.
Hope this helps. 342