Don't cut the steerer just yet. Set the bike up and install stem at highest or lowest position using spacers to take up the slack above or below the stem. Ride it and over a few weeks add or subtract spacers to get the desired stem setting. Then mark and cut. I always leave at least 10mm extra steerer above the stem for a "just in case" scenario, and spring time break in of the back to the position after a long winter break.