It is unclear when the father retired from the partnership. But in 1899, the Brooklyn Eagle reported plans filed by William M. Calder to develop the southeast corner of 7th Avenue and 11th Street, and there is no mention of his father Alexander:
Brooklyn Eagle, March 25, 1899, p. 14 ("Real Estate Market")
The property extended most of the way to 12th Street and included the large corner building and two additional buildings facing 7th Avenue:
It is interesting and instructive to contrast these 1899 mixed-use (flats over stores) buildings with Calder's earlier row of the same type at 7th Avenue and 7th Street, from 1888.
William Calder was apparently an efficient project manager, for the 1899 buildings went up quickly. Later the same year, in November 1899, the Brooklyn Eagle reported the sale by Calder of the two buildings fronting 7th Avenue between 11th & 12th Streets:
Brooklyn Eagle, November 24, 1899, p. 12 ("Real Estate Market")