Try grinding finer to slow the flow and stop pulling when you get the quantity you desire.

I think some people's definition of "ristretto" is probably pretty skewed. Having said that, if what you are looking for is something with more body and more oils, try pulling on the lever harder. Usually a thinner espresso points to lower pressure being put through the coffee. I can't remember the link to the post about lever pressure exactly, but I believe somewhere around 40 lbs should give you somewhere around 9 bar, but also pressure is relative to other variables and also quite difficult to measure accurately.

If you can't achieve this properly, I would first make sure your coffee is fresh (~3 days post roast) and then take a look at your grinder as tweaking the shots to varying degrees like this becomes ridiculously difficult without a great grinder. You may not ever NEED a really good industrial grinder, but sometimes to get the variation and qualities some people look for in shots, I happen to believe that you do. A "home" grinder should give adequate results and probably something very drinkable, but maybe not the shot you are looking for.

Also truly...ristretto means "restricted." So really just lowering shot volume will give you a "ristretto" given the same amount of coffee you would use for a regular shot. But the question is....do you really want a ristretto? Or do you want a drink that tastes it's best? Maybe you could also play around with the single basket...