Yang, L. et al. Permanent genetic memory with >1-byte capacity. Nat. Methods 11, 1261–1266 (2014).

In this publication, Yang and colleagues identified orthogonal recombinases by mining (finding by bioinformatics-driven searches) their sequences from prophage genomes. These recombinases bind to their cognate attachment sites (attP and attB) and flip or excise the flanked sequence depending on site orientation.
Even though the authors showed orthogonality and effectiveness, calculating a percentage of fluorescent cells after the experiments, a simple metrology to convey recombinase efficiency was not described. Additionally, reporter expression levels would depend on the specific sequence being flipped, adding a layer of non-recombinase based variation to dynamic range and other measures.
Thus, these parts have no attached quantitative values.