Well, I don’t like the word impossible, but let’s just say it’s difficult to find out how much memory would use an object that doesn’t yet exist, so, if rough estimations are not enough, then doing it after the object exists via the threshold approach seems more logical.
In case you didn’t yet check them out and they help with either getting the values or verifying if they are the correct ones, take a look here or here, maybe they’re of some use, in addition to inspecting things via Chrome Dev Tools.