Um... there isn't one.

Actually I drew many plans on CADRail, including a variant for MR layout planning competition.

But the plans served only to give a general idea of what is possible in the space. Now, under the influence of crew member Peter, I just build the platform with a general idea of the configuration, then play around with track to see what fits.

this is made easier by the collection of second-hand track I have, recycled from the original SPL and bought second-hand on TradeMe, the New Zealand equivalent of eBay.

For the original SPL, I planned the layout to the millimetre, then ordered the track. It worked, but in a confined space things never quite turn out as planned, so I ended up winging parts of it anyway, and other parts would have been better planned if I could see the result.

So now I lay down the basic shape via framing, then cookie-cut trackbed to a closer shape, then do final refinement as I lay the track.