It is interesting that there is no thorough discussion and clear derivation on this per my search. I know TIPS are complex (compared to nominal bonds). The naive use of simple spot/forward yield difference seems to be the "carry/rolldown" in real terms but not nominal.
My best hunch is that: assuming we ignore floor protection, and assuming we have built a breakeven inflation curve (seasonally adjusted) using whatever method. We fix 1) nominal market discount (treasury curve) and 2) all breakeven price indexes in the future, then we calculate the fair present value of TIPS as of today, then we recalculate the present value one year later, in which we keep nominal term structure the same, and breakeven prices indexes realized. The difference is then the total expected nominal value change of TIPS.
Any better idea?