The Collaborative International Dictionary of English v.0.48:

Proportionately \Pro*por"tion*ate*ly\, adv.
   In a proportionate manner; with due proportion;
   proportionally.
   [1913 Webster]

WordNet (r) 3.0 (2006):

proportionately
    adv 1: to a proportionate degree; "your salary will rise
           proportionately to your workload" [syn:
           proportionately, proportionally] [ant:
           disproportionately]
    2: in proportion; "the height of this wall must be reduced
       proportionately to give the room pleasant dimensions" [ant:
       disproportionately]
    3: in proportion [syn: proportionately, pro rata]

Moby Thesaurus II by Grady Ward, 1.0:

31 Moby Thesaurus words for "proportionately":
   ad eundem, appositely, as, as well, ceteris paribus, comparatively,
   correspondingly, distributively, each to each, equally,
   equivalently, even stephen, evenly, fifty-fifty, germanely,
   half-and-half, identically, in equal shares, in proportion,
   indifferently, not absolutely, pertinently, pro rata, relatively,
   relevantly, respectively, severally, so, to a degree, to an extent,
   without distinction