An analysis of survey data of buildings from the Liao, Song and Jin dynasties can serve to understand the height of dougong and the dimensions of cai and fen, which were the modular basis (cai-fen system) for timber-framed structures in that period. Based on the idea of “cai is the key of prime importance” for timber framing, and according to our knowledge about cai-fen grades in Song building standards, together with the heights of cai actually measured in palatial-style architecture (diange), it is possible to calculate the grades of the cai originally probably used in these buildings and to estimate the possible length of their construction ruler (chi). Taking Fengguo Monastery as an example, this article suggests the possible length of the ruler used in the construction of the main hall and clarifies the hall’s original modular design basis and its use in calculating the size of important building components. This then allows drawing conclusions about the possible timber grades and the possible ruler lengths of timber-framed structures built between the mid-tenth century and the twelfth century. By doing so, we can gain a deeper understanding of traditional Chinese architecture in that period.