Heseiba (reclining) is a Rabbinic ordinance of the Passover seder intended to invoke the style of derech cherut ("the way of freedom" - associated with luxury/aristocracy). Some Rishonim (medieval Torah scholars) opine that other mitzvot of the night are not considered valid without heseiba (e.g. the biblical commandment of eating matzah is not considered fulfilled without a concurrent, halachikally-valid heseiba; see e.g. http://www.koltorah.org/ravj/13-28%20The%20Mitzvah%20of%20Heseiba.htm).
Heseiba as commonly practiced generally consists of leaning slightly to the left in one's upright chair. However, the historical act of leisurely reclining-while-eating in the ancient middle east was much more dramatically distinct, consisting of reclining to the left with one's length more closely parallel to the ground. (See also: http://blogs.getty.edu/iris/reclining-and-dining-and-drinking-in-ancient-rome/ and http://sciencenordic.com/why-did-romans-recline-while-feasting )
Rabbi Yosef Dov Soloveichik is quoted as having required leaning upon some object in order to qualify as heseiba. However, do any halachic authorities discuss the shiur of the incline of heseiba? Considering that the modern prevalent practice is considered, if anything, uncomfortable, is the issue at all discussed in the context of the actual ancient rite of reclining-while-dining whether leaning slightly to the left in an upright chair qualifies as heseiba?
(I myself try to use a second [folding] chair with a pillow/cushion to accomplish something more comfortable that more closely mimics the old custom. I was wondering if the issue is raised at all in halachik works.)