see Plotho's history of the 1815 campaign, available on Google. Be advised there have been at least two scans and they are both rife with scanning errors so you may need to pull down multiple copies. I snagged two over the last couple of years and between them they are fairly complete - excepting the foldout pages, which Google never captures.
Look at Beilage (loosely meaning Appendix) VII, section 2) which describes the location of the units in the Brunswick Corps on 14 June 1815; Beilage X, subsection C.a which describes the organization of the Advance Guard Battalion; Beilage XVIII describes the losses, starting on p.76, with the Brunswick losses broken out on p. 94 by Quatre Bras and Waterloo.
At one time I had the actual battalion strengths immediately before the campaign started. At this time I cannot put my hands on that file. The "672" per battalion in Siborne and since copied elsewhere is either the establishment or an aggregate strength divided by the number of battalions. I've never looked into it. You may be assured that it is not correct for each battalion.
I hope this is helpful,