Today I received an email from dailyhalacha.com that has a sefardic take on this question:
Haham Ben-Sion defines haseva as leaning forty-five degrees to the left, and one must be leaning on something such as the back of the chair or the table, and not simply leaning to the side in mid air with no support. Leave enough room for this when preparing the seating.
The full section by Rabbi Eli J Mansour reads:
When and How to Recline
The Raavayah determines that since people no longer eat as was once common practice, namely close to the floor on a couch or supported by cushions, it is therefore no longer obligatory to recline, causing us discomfort instead of feeling like kings. Maran rejects this opinion and considers reclining mandatory even today when we are used to eating sitting up straight in chairs. We recline while drinking the arbah kosot and while eating the massa. If one does not recline as he drinks one of the arbah kosot, he hasn't fulfilled his obligation and must drink another kos. So, before making Kiddush, the baal habayit should make an announcement, reminding everyone to recline while drinking. Keep in mind that reclining, haseva, is always to the left. Each person should concentrate on reclining to his own left and not be confused by the people sitting opposite him who look like they're reclining to their right. The Sages offer two reasons for reclining to the left. One is for safety purposes, to avoid the sakana of our food or drink being ingested by the windpipe which might happen if we lean to the right. The other reason is practical. Since most people are righties and need their right hands to eat, reclining to the left makes their right hands available for holding the kos or the massa. The Rabbis decided that even a lefty should recline to the left, despite the inconvenience, in order to avoid the aforementioned sakana. Haham Ben-Sion defines haseva as leaning forty-five degrees to the left, and one must be leaning on something such as the back of the chair or the table, and not simply leaning to the side in mid air with no support. Leave enough room for this when preparing the seating. It is debatable whether or not women should recline, some opinions maintain that a woman shouldn't do haseva unless she is considered an isha hashuva, an important woman. Shulhan Aruch says that all women are hashuvot, important and significant, and therefore they should also recline."
I did not find a copy of this whole document posted on the web, so I quoted the relevant section above. The file is named "Pesah Seder Guide.pdf". Perhaps it will show up on the web at a later date.