I want to buy the anti-freeze as a concentrate and dilute it myself

We recommend using treated water (demineralised) in order to limit the introduction of compounds (chlorides, sulphates, carbonates) that can jeopardise corrosion resistance.  All the standardised corrosion tests are carried out with concentrations of 33% by volume of anti-freeze.  Below this concentration, the amount of corrosion inhibitor in the circuit is too low and may not provide adequate protection against corrosion.

That is why we recommend, not just because of the protection provided against freezing, using a minimum concentration of 33% by volume of anti-freeze.  This concentration, depending on the anti-freeze gives  freezing point of 12 - 18C.