The way this is implemented in your suggestion (and previously was implemented) is that there is an uncertainty of 50% on the W+jets sample (split into w+b, w+c and w+light). Additionally, there is an uncertainty of 87% on the w+c and w+b samples. So we've got a 100% uncertainty on w+c and w+b. 
This overcomplicates things though - we can just have one rate uncertainty per process. There is not necessarily a correlation between the rates for W+light and W+b or W+c.