It has long been known that foundry Al-Si alloys cannot reliably and consistently be grain refined by means of Ti-containing master alloys due to reaction between Ti and Si. Over the last years we developed and studied new Nb-based grain refiners consistently demonstrating the effectiveness of Nb + B inoculation on foundry alloys studying various Al-Si compositions solidified under different cooling conditions. Most of our studies focused on achieving the finest possible grain size using a lab made Al-2Nb-2B master alloy. This work presents the results, issues, and advancements made from the early stages of development to the first pilot scale trial. Moreover, the results of the present study are a further justification of the choice of the Al-2Nb-2B nominal composition, which was primarily dictated by the poor yield/recovery of Nb and B from their raw materials at lab scale. In the present work, emphasis is put on addressing the unanswered question about the effect of Nb to B ratio considering the inoculation of an Al-10Si alloys via the addition of Al-Nb-B master alloys with different stoichiometries.