I don't think you can make LR produce an exact file size. I just tried with a 12M pixel image, and setting 5M byte (which is approximately 5.243 kbyte, so I set "Limit file size to" to 5243) and it created a file of 3.9Mbyte. Well, that's within the limit I set!
Conversely, If I unchecked the "Limit file size" option and set quality to 100, I got a file size of 10.3Mbyte.
I think you may have to experiment.
Thing is, jpeg compression is a bit of a black art. When software uses an algorithm to compress an image, you don't know the size of the compressed file until you've finished. To get a particular size, you could try with initial compression parameters, then if the file is too small try again with slightly higher quality parameters, vice versa if the file is too large, and iterate until you get the right size. The software would have to do this with each image (as the compression you get depends on the image content), and this would make the jpeg compression rather slow.
I guess LR just takes a conservative estimate of compression parameters that are pretty much guaranteed to be within the file size limit you set, but it might be quite a bit smaller.