I would get a block of wood (or anything that is flat enough and the right thickness - i.e taller than the highest part of the bit you want to cut off) and tape a sharp pencil to it.
Then you have a fixed gauge with which you can mark the bottom angles of the legs, either directly off the floor (if you're confident that the floor is flat), or preferably from a known flat surface like a planer bed or some other bit of machinery with a nice flat machined bed. If you use the same gauge for all the legs they will all be the same length.
Then saw them off with a handsaw to the line; I recently did 24 baby chairs this way and it was much quicker to do by hand than muck about making jigs and whatnot, plus it covers up the inaccuracies in the chairs themselves..
A jig will cut all the legs at exactly the same angle, that's great, but only if all the legs are exactly the same angle, which they almost certainly won't be, so a scribing gauge like this will make them all correct in relation to the floor, which is what you want.
Then, if you like, you can add felt pads to the bottom of all the legs which will take up any minor (sub millimetre) inaccuracies in your cutting.
I hope that helps.