Yes, that is a total clearance between the rocker stands by pushing both rockers and the spacer to one side and measuring.
Harland used to provide flat ground spacers to use so that they would line up with either the closer valve spacing of most all FE heads, or the wider valve spacing of the MR, HR and TP heads, depending on where you put the spacer. Not sure if they are available separately or not. I used to use valve spring seats, the flat ground hardened type, when I was young and used what was available. They fit the shaft perfectly. If I recall correctly, they were small block chevy pieces, but I won't swear to it.