𝜆 in Eq. 4

Hi, thank you for the interesting paper. May I ask a question here?

how do you know the $\lambda$ in Eq. 4?

The footnotes says $\lambda=0.5$ for DDPM. Would you like expand on the reasoning behind this? Or just trying different values of $\lambda$ until it works if it is used on a backdoor attack on DM? Thanks.