Coffee questions for Mr. Coffee Geek...

I'm very happy with the base settings of my new Breville dual-boiler plus Baratza Vario combination but now I'm ready to play. 

Currently using the default setting for 200°F on the espresso machine, and grinding at 3/K for 10 secs on the Vario. I just bought 8oz of Sumatran single-estate beans french roasted today. Fabulous crema. 

My question: how should I go about trying different temperatures and grinds. Where would you start? Why? 
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