You can't set amount to be lower than 1 in industry tab (Economics view). Of course you don't want to build half a research lab,but that cause issues in some scenarios :
If you want to build 2 RL, then if one month later you want to stop at only 1 RL, you can't. Amount will now be at 1.90 , and since you can't modify that to 0.9, you're screwed.