The problem: I need to reverse my car into the garage, in order that i can get out of the drivers door.

This used to be a bit hit and miss getting into the right spot, and when i had parked there was always the questions...;

Have i come far enough in so the garage door can close?
Is there enough room to walk past the back of the car and open the boot?
How close am i to the sides - can i get to stuff with out moving the car?

so i devised a simple and quick parking aid to sort this once and for all:-)

What you will need

Car with a reversing camera and backing grid (picture1.1)( although i am sure you could use a part of the car as a reference and adapt it to mirror only use?)
Garage with a concrete floor
Can of road marking "fluorescent orange paint"(picture1.2)
Two boards approx 1m long with one straight edge at least
Broom
Painting mask - this paint is vicious - (open all the doors too)

Step 1: Layout

First is the hard bit...

Back the car as in as straight as possible so there is even clearance on your mirrors until you find the sweet spot that suits your situation, ie;

clearance for the garage door
clearance to open the boot or get by
clearance around the sides as required

Next,

Reverse the car to the best parked spot,(easiest with helper) so that you have just enough clearance for the garage door to close, and what ever clearance you require at the back.
Choose a point of reference on the rear of the car, in my case its the center mounted tow bar.(visible as the blob in center bottom of my camera photos) and identify your cars reversing limit line(picture 2.0)
Using the broom sweep the floor thoroughly in the area behind your car to remove all dust.
Lay out a board parallel to just behind the car.(picture 2.1) (center it on your reference point) and adjust its position (by trial and error) until your camera limit line is parallel to the rear edge of the board, closest to the back wall of the garage.
When you have it lined up, lay a second board on the garage rear wall side, parallel but with a 50mm gap.(picture 2.2)
Now use a marker pen to mark the position of the of both boards so they can be replaced exactly for painting. (position 1)

Next,

Now remove the first board you used to line up with the reference line, but leave the second board in position.
Lay another two boards at 90 degrees to the line remaining board, so they are centered on the reference point you choice on your car.(picture 2.3)(position 2) and mark their position so you can replace them later.

Step 2: Painting

Remove the car from the garage and open all the doors, put on a painting mask and eye protection(this paint is very stinky and fills the air)
Remove the boards form the previous step and then replace the missing board onto the position one markings (closest to the front of the garage)
Paint the gap between the boards - so you have a 50mm x 1000mm line
Let the paint dry
Re position and line up the boards with the marks for position 2 then paint this gap. You should now have a second 50mm x 1000mm line at 90 degrees

Now you should have a capital T painted on the floor - the cross piece parallel to the rear of the garage, the "stalk" pointing out to the garage door.

Let the paint dry, then remove the boards. (picture 3.1)

Step 3: Parking

Right this is the simple bit;

Jump in your car, and reverse slowly in assuring equal clearance on the mirrors
Then line your reference point up with its line(position2 - the "stalk of the T") and reverse back until it is over the top.
Then its a matter of simply backing the rest of the way until your cameras limit line touches the front line you painted at position 1 (picture 4.0)
You should now have clearance to close the garage door and move around the car as you desired.

Step 4: Refining

After a few months of using the line system.. i added a refinement;

Using a couple of long boards - I extended the line pointing towards the garage door ( "stalk of the T") all the way out to the garage door.
Now when i back in ( using the reversing camera) its as simple as putting my reference point ( the tow bar) right over the line and keeping it there as i back in..