If I remember correctly there are 2 different legal measurements for headlight aim which depends on the height of the actual headlight and the "DIY" method requires tape and roughly 4 meters between the wall and headlight for a decent adjustment..
The following is a snippet I found online which describes the process.
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To check and set the alignment properly you’re going to need a friend, and a ruler or tape measure. Get a friend to sit on the bike to hold is upright, then measure the distance from the ground to the centreline of the bulb.
This is important as different rules apply depending on the bulb height I.e up to 850mm from the ground and ones that are over 850mm from the ground.
Create a mark on a wall or garage door, at the same height as your headlight bulb is from the ground. This line will be called the “horizontal zero percent line”. Next, draw a vertical line up from the ground crossing the “horizontal zero percent line” and extending about 300mm passed it. This line is the “vertical zero percent line”.
Start with the bike’s front tyre touching the vertical zero percent line, then wheel it back just under 4 meters (3.81 meters) away from the wall. Now have a friend sit on the bike in the normal riding position. It’s important to ensure that your tyres are correctly inflated as this will affect the reading.
Bulbs below 850mm from the ground
If the centre of your motorbike’s headlight bulb is less than 850mm from the ground, then with your bike in the position described above, the top projection of the dipped beam should be between 20mm to 80mm below the horizontal zero percent line.
Bulbs above 850mm from the ground
In this instance, with your bike in the position described above, the top of the dipped beam projection should be between 50mm and 110mm below the horizontal zero percent line. If your bike is within the tolerances then the next step is to check your beams positioning against the vertical zero percent line. If not, adjust your headlight beam until it does fall within tolerance. Most motorcycles have a knob at the back of the headlight unit allowing for easy adjustment.
Vertical zero percent line
All UK specification motorcycles should be designed to be ridden on the left hand-hand side of the road. In lighting terms this means that the light from the headlight should not aim to the right, potentially dazzling oncoming vehicles. Most UK bikes will have a headlight that kicks up to left. It is important that this kick-up doesn’t start immediately from the vertical zero percent line. To test this MOT technicians will use what is a called a 2% line, but on your wall or garage door, just ensure there is a gap of around 100mm between the vertical zero percent line and the area where your headlight starts to kick up.
If your headlight doesn’t have any kick up at all that’s OK. If it kicks up to the right, or direct from the vertical zero percent line, then you can use a shroud or a mask to hide this. This should only be a temporary measure, but make sure it doesn’t cover too much of the headlight, as it’s at the MOT tester’s discretion as to whether it unduly reduces the light output from the unit.
Although it might sound complicated, checking your motorcycle headlights for correct alignment is relatively simple. It only takes 10 minutes, and could be the difference between an MOT pass or a failure.
