Brix measurements in Copper Sulphate solutions

Brix gives a solution SG. I have a table giving relationship but no formula offhand, and it isn't linear, 0 Brix = 1 SG, 100 Brix = 1.56 SG, but 50 Brix = 1.23 SG. Table is from a Worthington - Simpson (remember them?) doc dated Nov 1965, but the data might be on Wiki.

The CRC Handbook of Chem & Physics gives CuSO₄ concn in terms of solution SG, page D-226 in my 57th edition. If you don't need great accuracy, A% w/w CuSO₄ and SG are related by SG = 1 + A/90. eg when A = 18%, gives SG = 1.2. Table says 1.2059, when A = 9%, gives SG = 1.1. Table says 1.0955.

Thanks!