Correct spelling for the English word "Customably" is [kˈʌstə͡ʊməblɪ], [kˈʌstəʊməblɪ], [k_ˈʌ_s_t_əʊ_m_ə_b_l_ɪ] (IPA phonetic alphabet).
Usually.
Etymological and pronouncing dictionary of the English language. By Stormonth, James, Phelp, P. H. Published 1874.