There isn't any....protection is to the C3's 60 dbu contour (which will be around 20 miles out on flat terrain).
Look at Section 74.1204--the translator's 54 dbu (50/10) can't overlap the C3's 60 dbu (50/50).
(Yes, there is the barn-door loophole about interference occurring over unpopulated areas, but if you try that you may run into to someone like me who will fight you tooth and nail. Had some idiots who tried to put a translator on 94.3 inside the 65 dbu contour of my 93.9 station. Never got accepted for filing).
If you go to Search>FMquery, look at the top of the page, there are links to several useful tools. One will give you the distance to a contour if you enter in the ERP, coordinates and AMSL of the antenna.