On an even plane
smoothly, regularly, without bumps, uniformly, unvaryingly, steadily, constantly, placidly, tranquilly, fluently, on an even keel, in a groove, without variation, neither up nor down, levelly, straight.
Antonyms irregularly*, unevenly, roughly.
Equally proportioned or distributed
exactly, justly, fairly, precisely, equally, impartially, identically, equitably, symmetrically, conformably, commensurably, commensurately, proportionately, synonymously, analogously, equivalently, correspondingly, tied, alike, squarely, fifty-fifty*, even-steven*.
Antonyms inequitably, unjustly, unfairly.