Whatever you do at this stage in your journey harley stick to one tuning!Open G is good advice.Glad you got the Cleartune app. As you obviously have to pitch your own notes in any style of slide playing it really helps to be in tune to start with.
I picked up a National Chicagoan awhile back to mess around with. It's really a lot of fun to play. I can get MUCH better tone out of it holding it Spanish style and playing with a slide. Looks weird I know, but works for me. Does anyone actually play these things like that? Is there any major disadvantage?
Sometimes you're the windshield, sometimes you're the bug!