As a comparison, iOTL, Reagan won California 52.7 to 35.9 (so by roughly 17 points.) Assuming that he *barely* wins California iTTL , let's assume he does 16 points worse across the board (I know that there will be a variety of differences especially with Carey not doing as well as Carter in the South, but just considering..). That means he wins (in order by Margin) Utah, Idaho, Nebraska, North Dakota, Nevada, Wyoming, Arizona, New Hampshire, South Dakota, Alaska, Oklahoma, Kansas, Montana, Colorado, Indiana, New Mexico, Florida and California.)
This actually gives a fairly simple map with Carey taking everything east of Kansas City Missouri except for three states: Florida, Indiana and New Hampshire and Reagan taking everything west of Kansas City, Missouri except for Texas, Oregon, Washington State, and Hawaii. Final in EV in this case would be 394-144.
And *very* oddly, most of the core south: NC, SC, TN, AL, MS and AR were almost identical in the race with Reagan winning iOTL by between 0 and 2.2 points. So even if Carey does 3 points better than OTL he keeps most of the south .
Note, Illinois is the determining state here. Reagan won it by 7.9% iOTL. If Reagan does 7.9% worse than OTL, it is 257 Carey, 255 Regan with Illinois's 26 EV determining the winner.