There is no guaranteed amount that will make it safe, it really depends on what you are using as your stabby tip material. We use Gold's Gym yoga mat as a material for our stabbing tips, instead of open cell. This is because open cell breaks down easier and tends to tear apart. Layering gold's gym yoga mat in 2-3 layers usually yields good results and passes safety tests. 2 layers for a blue, and 3 for a spear or red weapon.
-Tooka