Qualifications: I actually use these in my job and calibrate them with multiple solutions. In my job they are used to test SG of urine and is regulated by CAP (college of american pathologists).

As long as your reference fluid is accurate, then you can use it to calibrate. Your RO/DI should be pure, but might not. Ideally your refractometer shouldn't change day to day, but it can. Ideally you want to batch make your RO/DI water, then measure it and bottle it in a glass bottle so that the value doesn't change.

Get your LFS and yourself to make sure the RO/DI has nothing in it to verify. Once that value of water is measured and you have the true value you can pull from that bottle as often as you need. Don't pull from your RO/DI device as that value can change day to day.

In my job we use a calibrated solution of Saline water, pure water and two different urine specimens with calibrated values.

for the quick answer- IF you know the TRUE value of the solution SG, then you can use whatever you want.