There are two questions, both of which are complicated.
How much does specific gravity actually change for a fixed salinity as the temperature is changed. I expect this one does not interest you, but it is a real effect.
Chemistry and the Aquarium: Specific Gravity: Oh How Complicated! ? Advanced Aquarist | Aquarist Magazine and Blog
https://www.advancedaquarist.com/2002/1/chemistry
Assuming you are defining specific gravity as the ratio of the density of the seawater to the density of pure water at the exact same temperature, then the specific gravity of natural seawater (S =35) is 1.0278 at 3.98 °C, 1.0269 at 60 °F, and 1.0266 at 20 °C, and 1.0264 at 77 °F. The article above explains how these are arrived at.
Then there is the issue of how the refractive index of seawater changes with temperature, and how (or if) refractometers compensate for that.
The refractive index of seawater as a function of temperature, pressure and two wavelengths
https://www.sciencedirect.com/science/article/pii/0011747171900507
At 632.8 nm wavelength (reddish), the paper above shows the refractive index of 35 ppt seawater has these values:
0.03°C..........5.03°C.......... 10.03°C.......... 15.02°C ..........20-00°C.......... 24.99°C ..........29.98°C
1.34015... ...1.33977 .........1.33935......... 1.33899 ............1.33850............ 1.33795 .........1.33737
Thus, if you do not correct at all, the difference between 20 deg C and 30 deg C (a big temp change) is 1.33850 to 1.33737
That difference of about 0.00113 equates to the equivalent of about 5.6 ppt, the the specific gravity difference from about 1.023 to 1.027.
But you cannot make the correction yourself without knowing what corection the Milwaukee is already using.
