Already did that, using the constructors method ( Pythagoras theorem ) and it is almost fine but not completely, also seems hard to achieve an absolutely flat rail, so I thought that I may be missing something in the fine print which can come in the diagram.
I would get me a long levelling slate and a spirit level and make one rail straight and level. Then I'd adjust the second rail in regards to the first by laying the spirit level transversely over both rails. Then finish off with controllling the second rail to be straight.
Yes, trial and error, though with some system to it.