Can I ask tip for tissue measuring(especially for RNA preparation)?
I do not have specific experience with liver tissue, but here I would do a couple of things. First, to get around your issues with weighing tissue, set an acceptable range around 20 (say, plus or minus 5mg, so ranging from 15 to 25mg) and then normalize your RNA yield to that starting weight. Second, look into ways to preserve the RNA in your tissue as you're processing it. You could try RNAlater solutions or other chemical means of stopping RNA degradation, or just snap freeze your tissue in liquid nitrogen. Good luck!