My experience may be different from others. I'll say what I know to be true for me. Form is relative like heavy is relative. Every time I get under the bar I'm working form, there is nothing else to work…when the form is solidified and good, and things are gettign easy then it's time to move up in weight.
Form is never down, it is never done and lifting an empty bar is not the same as 40 kilos or 100 kilos or 200kilos.