I don't know of many Prime Ministers in BT, but I'd assume on that scale that 15 is several planets.
Question to answer your question, how relevant is it for your campaign? Are you planning to allow players that own planets?
If it's in relation to an NPC, I'd say not to bother with it. If NPC Bob is a Prime Minister who happens to own several planets, getting the "correct" TP for Property isn't real important in my opinion. He owns several planets, end of story.
If you're running an unusual type of campaign where your players can own one or more planets, the typical XP scale of things may not apply.