Log Cabin Base

As you can see from today's blip, I have been working hard on preparing the base for the log cabin...

As you can see it takes up a fair bit of the back garden. I have decided to use nice heavy timbers. They are a similar size to the timbers which formed the joist for the wharf for the old ferry crossing of the river severn!

Actually I have bit of a problem.

The base obviously needs to be perfectly level. It also needs to be perfectly square. The cabin has no frame, the logs interlock on each other so squareness is vital.

Now I have laid out a 12m squared rectangle, all the sides are parallel. But the 2 diagonal distances are different. I have a parallelogram, rather than a rectangle. Not a right angle in sight. Mr Hypotenuse will be turning inhis grave.

So my dimella is which corner to 'squish?' How do I make my rhomboid shape rectangular???

Now in the calm atmosphere of my sun rooom I think I may have figured it out. But out in the garden in the heat of the battle with the sodding wood, gravel, spirit level and slabs I must confess to more than momentarily losing my cool.

UPDATE: How can you have 3 sodding right angles and still be out??? This is like shortening the legs on a table to get it level. Bad light has stopped play, glass of wine and a fresh look in the morning.