I've just completed the 3-month BBB Challenge using Front Squats (unfortunately standard Squats just mess up my back, hence the FS alternative). The 5x10 at 70% in the final month were ... interesting.
Results were exceptional, perhaps as good as I've experienced using any of the templates from Beyond and the original 531--albeit 10 reps is as high as I'd ever go, even with such a moderate percentage.
As Jim stated, 5s PRO seems to be the best option if you FS as the main movement. For PR sets above 6-7, it might be a struggle.