I understand your question is about using the leavers to draw conclusions about the whole. My point was to urge caution selecting the number for your denominator.
In order for you to move ahead, given your date, the denominator should be total effectives (on the rolls) excluding officers, not simply present as it does not include others (that would be what Siborne should have used as he was calculating battle strength).
To be clear, it may be that Siborne used the total effectives number (or, as he did with the Brunswickers, the establishment strength) and Petit some other count (at 1040 men for the 1st Grenadiers on 15th June 1815).
Without doing lots of work, my understanding is that the 1st Grenadiers had 41 officers and 1,239 other ranks present under arms on 16 June 1815. This count excludes detached men (for whatever reason behind their being detached) and men in hospital as well as any prisoners with the enemy (doubtful at that date).
Thus, Siborne's number of 1500 may be the correct one for you to use as a denomiator, if - say - 100 men were detached (even for a fighting task, so long as they were no longer with the unit) and 161 are in hospital. Those numbers are invented to make the difference work, but they are possible (near to hand I have the 30 October 1806 numbers for the 1st Grenadier Regiment when it had 1,703 present, 83 detached and 43 in hospital or 1829; the 1829 would be the correct number to use for your purposes).
From my years of looking at returns, it may be no one is on detached service, yet I rarely see returns without anyone in hospital.
So, again, my point is to urge you to think through which is the correct number for the denominator. Looking at the numbers you gave, for statistical purposes, it would be better to use Siborne's number, rather than Petit's number. - R