Please recommend a standard conversion chart/online tool for converting American measurements to the Metric system.

I use an app called Convert Any Measurement. Very accurate and it's free.

I always use this guide: http://www.kingarthurflour.com/learn/ingredient-weight-chart.html

I bought the Baker's Appendix from Food52 a few months ago and I have loved using it. https://food52.com/shop/products/1265-the-baker-s-appendix
There are charts for different types of flours and sugars. I use it every time I am measuring something that I want precision. Highly recommend.