Product family design and platform-based product development have garnered much attention over the last decade. They have been used to provide nearly customized products to satisfy individual customer requirements and simultaneously achieve economies of scale during production. The inherent challenge in product family design is to balance the tradeoff between product commonality (how well the components and functions can be reused across a product family) and variety (the range of different products in a product family). Quantifying this tradeoff at the product family planning stage, in a way that supports the engineering design process, has yet to be accomplished. Responding to this need, we have developed a graphical evaluation method, the Product Family Evaluation Graph (PFEG), that allows designers to choose the 'best' product family design option among sets of alternatives based on their performance with respect to the ideal commonality/variety trade off, given a company's competitive focus. One of the necessary supporting pieces for the PFEG is to develop a quantitative representation of the ideal tradeoff between commonality and variety in a product family based on the elements that characterize a company's competitive focus. In this paper, we develop the commonality/variety tradeoff angle, α, ranging from 0° to 90°, as a quantitative representation of the ideal commonality/variety tradeoff in product families based on a company's competitive focus and their industry-wide competitors' information. α is defined as a function of the weighted sum of the strategic factors' quantitative impact on commonality and variety (S) in a product family. These factors cover five categories - market, product characteristics, life-cycle processes, government and industry regulations and/or standards, and organizational capabilities. In this paper, we analyze whether each of these factors causes an increase or decrease in commonality or variety to better understand their impacts. The factors and their categories are admittedly incomplete and the analyses of the factors' impact are subjective. A more accurate factor identification and impact analysis will be necessary in the near future as the number of companies using the product family strategy increase. We intend for this paper to serve as a basis for that expansion. In this paper, we propose a three-step approach to estimate this angle for a given company using the linear regression model. The angle can then be used with the PFEG to help designers evaluate a product family or compare product family design alternatives. The proposed angle is illustrated with an application to four families of power tools.