Parsing inflected forms may not always work as expected. If the following does not give the correct word, try Latin Words or Perseus.
prōportĭōnālis, e, adj. [proportio], of or belonging to proportion, proportional (post-class.): termini, Front. Colon. p. 111.
Hence, prōportĭōnālĭter, adv., proportionally, Cassiod. Geom. 6.