The "fuzzy line" phenomena of reading a refractometer is due to a cloudy sample. Clear wort before yeast pitch, no problem. Clear beer after fermentation and settling, no problem. Murky beer during fermentation with lots of solids, problem.
Solution? Use a settled, degassed sample. That's the only accurate way to get a valid reading. Which, coincidentally, is the only way to get a valid reading from a hydrometer. Also with both devices, accuracy is dependent on calibration of the device and temperature correction of the sample. During the brew session I take a shot glass sample of wort, agitate it with a pipette and chill it in the freezer to bring down the temperature to 16C/61F and precipitate solids. Takes about 10 minutes. During fermentation I take the sample and stir vigorously for 20~30 seconds to degas the sample and raise the temperature to 16C/61F.
Clarity makes it much easier to get rid of the fuzzy reading of refractive index. And remember, you are reading refractive index of the sample, measured as brix (glucose), not specific gravity and not the primary sugar found in wort (maltose). That's why you have to factor in a wort correction factor which is commonly 1.04 (4%). From there you merely consult a chart to obtain an equivalent specific gravity value, many of which already include the common wort correction factor of 1.04.
In addition, if alcohol is present you must also account for its influence on refractive index. There are many online calculators that do the math for you, but be aware that there are two different popular algorithms used in these calculators. One gives more accurate values for lower gravity worts (<1.060) and the other for O.G. >1.060, though at the end of the day the differences aren't significant for home brew level accuracy. If that's what you need, go buy a gas spectrometry device for a couple hundred thousand $$$. Trust me. The process may sound daunting and cumbersome and requires using "numbery" stuff. But hey, I'm a guy who barely made it through calculus (more than one attempt!) and if I can do it, it ain't that hard.