That's a pretty general rule most individuals aren't sure a what speed it applies to, and .5 MPH change with six colors out can easily make 9 feet differences in depth. Plus some brands sink more than others and higher test doesn't mean it will sink more. When I started testing depth results I was surprised at the outcomes. Lead core sink does vary with speed. I use sufix performance 18 lb test most of the time.
I've Fished for trout and bass with Lead core for some years. Trolling with lead core is basically the only type of fishing I do now. Lots of fun, and easy relaxing. Some may fine this useful. Lead Core Depth Calculator "https://play.google.com/store/search?q=fishing%20lead%20core&c=apps"c=apps