Sizing
Brix is percent sucrose. Your size box does not contain sucrose, and it is not at 20 °C — both corrections point the same way.
— %
Temperature-corrected and converted from Brix
The conversion factor is specific to the size recipe and must be established by oven-drying samples against their Brix — a starch-heavy mix and a PVA-heavy mix do not share one. An instrument with automatic temperature compensation applies its own correction, so set the coefficient to zero rather than correcting twice.
Sizing Brix to Solid Content Converter — free, with the formula and a worked example, at Textile School.