what's the recommended prusik length for the 8.5 tether?

It does depend on the diameter of the climb line you have and the type of hitch you tie. If you tend to use a more compact hitch a 26"-28" eye to eye will work great. If you tie a longer hitch like a VT you might need a 30"-32". Tying anything more than this length would require the longer tether most likely.