Three reads before you pay up for a tilt.
01
The premium is a spread. Long the cheap, the small, the winners. Short the expensive, the large, the losers. The premium is the gap between those two legs. A fund you can buy owns one side of it, never both.
02
How much survives depends on the factor. Size travels long-only almost whole. Momentum leaves half its premium on the short side you cannot reach. Read which factor you are actually buying before you assume you got all of it.
03
It is market beta plus a slice. A long-only factor return is the index plus a fraction of the spread. Separate the two. If the tilt adds two or three points, fine . just do not pay a long-short fee for a long-only slice.
The textbook premium is the gap between two legs. The fund you can buy owns one of them. Read which factor survives the trip from long-short to long-only before you pay up for the tilt.