I've developed a formula (ppt available for download here) from the OECD Secretariat's Unified Approach, applying the fixed routine (10% and 20%) and market share (20%) numbers the OECD used by way of example in its Impact Analysis, and plugging in the KPMG percentages for Amounts B and C.
The KPMG Study provides a starting point to consider how firms are likely to approach Pillar One and how nations are likely to fare under the OECD Secretariat's approach, now endorsed by the Inclusive Framework. Briefly, the study is described as "a fact-based economic analysis using comparables data of the arm’s length returns to sales, marketing and distribution," prepared by KPMG under commission by Microsoft. It looks more like a standard transfer pricing report than an actual economic analysis, and it raises the usual pile of questions about assumptions and definitions but it is a starting point for understanding the scope of possible redistribution under Pillar One. Like the Study and the OECD Impact Analysis, I've made a bunch of simplifying assumptions and would benefit from comments and suggestions.