When gybing, typically you are going from 120 deg apparent wind (from your aft quarter) on one side to 120 deg apparent wind on the other side. Due to the nature of angles between apparent and true wind, this equates to about a 90 degree turn overall. So pick a point on land (or a cloud) that is 90 degrees away. This is your target new heading.

If you’re doing a gybe single-handedly, first lock the wheel or set the autopilot so you can leave the helm station.