private int calculateNumIndexesLodDiff(int lod) {
        if (lod == 0) {
            lod = 1;
        }
        int length = getWidth() - 1; // make it even for lod calc
        int side = (length / lod) + 1 - (2);
        //System.out.println("side: "+side);
        int num = side * side * 2;
        //System.out.println("num: "+num);
        num -= 2 * side;    // remove one first row and one last row (they are only hit once each)
        //System.out.println("num2: "+num);
        // now get the degenerate indexes that exist between strip rows
        int degenerates = 2 * (side - (2)); // every row except the first and last
        num += degenerates;
        //System.out.println("degenerates: "+degenerates);

        //System.out.println("center, before edges: "+num);

        num += (getWidth() / lod) * 2 * 4;
        num++;

        num += 10;// TODO remove me: extra
        //System.out.println("Index buffer size: "+num);
        return num;
    }
calculate how many indexes there will be.
This isn't that precise and there might be a couple extra.
jme3-terrain/src/main/java/com/jme3/terrain/geomipmap/LODGeomap.java