Hi,

This is how I'd do it in a garage.

1) Firstly turn the boat over.

2) Measure and mark the centre line on the base transom with a pencil, so you can see it also on the bottom of an up turned hull. You can often do this just by measuring to a mid point.

2) Go to the front of the boat, work out where the centre line of the bow is and shine the laser beam through the points of the front and aft centre lines. With the board down (sticking up in the air), you can check the actual alignment of the centreboard and adjust if off centre, to match your laser beam centre line going from point to point. It helps if you get a mate /  partner / responsible child to help with the alignment of your laser line. As a bit of common sense, use your judgement to gauge if your laser centreline is on the centre, I wouldn't guarantee that your transom  is perfectly square to your hull and that may have affected transom centre line measurements initially.

3) Now you know your centre board is alligned to the centre of the boat, you can level the boat. If you simply want your rudder to be aligned to your centre board, all you need to do is make sure your level is 90 degrees to your board. (Assuming your board has no play!)

4) Once you have achieved step 4, you can go to your stern, mark up 90 degrees downwards from your premarked centreline and fit your pintell.

This is the quick way to do it, for more accuracy, you could attempt to level your boat  and ensure your centre board is square to the underside of your levelled boat. The problem is you have to be quite careful when levelling a boat, on the boat yard we use auto-detecting surveyors laser levels and spend a good bit of time working on it as everything needs to be perfectly square and level for fit out. There are ways of doing it with long cross bars or assumptions that deck edges crossing transoms are level, but for a semi reasonable quick do-able thing, steps 1-4 suffice.