I have mentioned valve cores in a number of posts. I even offered some advice on how and why you should avoid over-tightening the core Here.
Last week, I learned about a low cost tool that I feel I can endorse as I purchased and tested this tool and was pleased with the results.
This is essentially a "clicker" type torque tool. You tighten the valve core in a smooth twist and it will click and release when the core is tight.
Using my calibrated In-Oz Torque wrench I confirmed the Slime unit worked at 3.5 In-Ozwhich is in the range of 2 - 5 In Oz as specified by the Tire & Rim Association, the organization that publishes dimensional specifications for tires, Wheels and valves as used world-wide.
It's the Slime Part # 20178. I got mine at a local auto parts store but I see it listed as available at a number of stores and on-line.