In short you are counting the positions in the wrong order. Always start from the Big Blind to the right. This is exactly because the max players at a table can change. A game can grow from 4 to 10 or go from 10 to 4 players and will always need the Big Blind.
The definition I suggested will work only for the Hijack in a 6max table since there is only one MP in that table size. BB ; SB ; BTN ; CO ; MP (hijack) ; EP. Even if it has marked to use the three avaliable MPs for a 9max table.
Only if we wanted the speciic stat RFI Hijack on a 9max table we would need to edit that definition to only use the MP3 position.
In your specific definition change the Under the gun for the MP3 position and you should have a definition only for Hijack as you wanted
Hope this helps you understand how to look at the positions when editing the definitions and where to start counting them. Always start from BB!!