More like 70/30 skimmer/main drain. Bias the valve towards skimmer for good skimmer performance. So it depends on plumbing setup and you should not follow some fixed setting.
Some new pumps come with pump unions (Hayward, Sta-Rite, Pentair Super Flo, Waterway are the big names). Others, like the Pentair Intelliflow do not. For those a "hi-temp" pump union should be used. If you are going to use a 2-way valve get 2-way Jandy or Pentair diverter valves. If and when they need parts, including the diverter assembly itself, they are easy to obtain and repair. The parts are interchangeable with their 3-way valves.
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