When I redid mine, I took measurements but found it wasn’t all that necessary as everything fell into place. When you seat the new inner panels against the shock towers and clamp the radiator support pieces in, the back edge falls where it wants to be. When I measured it, it turned out exactly like the old ones prior to removal. Then I held the brackets to the cowl and marked the positions.
Even if you were off 1/4”, there is so much room to move the fenders around that I think you’d have no trouble getting everything lined up well afterwards. If you really wanted to be extra cautious, you could spot the inner panels to the shock towers, clamp up the front radiator support pieces, set the fenders on with a couple bolts, then measure the opening and compare to your hood.
