There is no rough estimate as it would be different for every company and completely dependent on the number of visitors that have their browser set to block or not run JavaScript. Once you figure out what this is for your business you would be able to use that figure.
I work with some pretty big companies that use GA and we don't worry too much about the discrepancy as it is often so small it doesn't really make a difference if you are looking at right numbers anyhow.
Basically, to get your overall picture of how your website is really performing (meaning how effective it is at turning website visitors into sales) you should be taking averages over a given range of time anyhow. Those averages when compared with previous numbers over a similar time range are what is going to determine your next move.
If the range is too large your numbers will be watered down ... if too small, they will have greater margin of error.
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