As far as counting frames goes, I do a bit of programming myself, so let me clear this up. If humans were computers, our eyes would update at around 50 FPS. Put simply, you cannot count frames, even if you're M2K. Knowing almost exactly the frame data and getting a really good feel for how it works is a great option, but the fact that most video games look so smooth is because they actually update a little faster than the human eye can see.
As far as knowing whether or not you L-Canceled goes, go grab Bowser. I'd suggest going into an infinite time match so you can see it exactly, but take him and Short hop -> bair. Watch how painfully long he takes to roll back onto his feet and think about the 50 billion possible punishes during that roll. Now SH -> bair and press the L or R button right as Bowser hits the ground. If he's still taking a painfully long time to get back up, you messed up your timing. Keep trying it out for a little bit and you should be able to figure it out. After that, switch over to your main and try to apply the same idea. Try right as your character lands, it seems to work for me. If you need the L-Cancel for your SHFFLs, I wouldn't suggest doing them in place. Here's how I taught myself to SHFFL after getting a feel for all the individual concepts.
1.) Start running across the stadium, consistently short hopping.
2.) Once you get a good feel for that, add in an aerial to it and practice that a little.
3.) Next, add in the fast fall to the rest of it.
4.) Finally, L-cancel your aerials. I would recommend seeing how quickly you can get back to dashing after the aerial without an L-Cancel first so you'll know if you reacted quickly enough to get the L-Cancel itself.
It's a bit tedious, but it worked for me.
