Petit gives the strength of the 1st Grenadiers as 1040men on 15th June 1815. a figure lower than Sibornes estimation of 1500.
So we have:
15th June 1040men
11th September 870
A loss of 170 men between 15th June and 11th September 1815.
Of the 870 men, I have data for 25% which works out as 22% of the total regiment starting the campaign. So the issue is, how repsentative of the whole is this %?