With regards to point 3 about the spillover being 25-32mm above the water line. What is the spillover level on the siphon? As said I haven't fit this type before.
Spillover point thus marked. Get the bottom of the siphon neck, (as marked), at the required distance above the water line, high enough so it doesn't constantly overflow, but low enough so the water doesn't escape from any other point on the cistern and flood the place....
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