Or if no 20XX, in regards just to AC fair, you'll see your "landing floor effect" when u land, either showing white smokey dust meaning you didn't AC or white pointy spikes (with a little dust), meaning your AC was successful.
As far as you feeling your L-cancel & AC timings sway off, I think you are actually noticing something, it's just not percentage based. You might be noticing the timing's alter & vary when you A.)hit them or B.)hit their shield. Which it is actually true you'll need to do completely different timings for LC depending on whether your hit will hit their shield or no shield.
This is the solution proposed by a Kadano video but I forgot which: Since you need to input L within the 7 frames before you land to succeed L-canceling, & since L/R has a downtime where u can't press L/R again (i forgot the numbers), you have to press L twice while coming down with your attack. I think it was 5 frames apart, at very least pretty quick. From my experience it has been: press L 1st before "hitting", since u don't kno if their shield will be out or not, then pressing L a 2nd time half way before reaching the ground. This usually covers your L-cancel differences with hitstun from hitting shield to hitting a person, sealing your L-cancel regardless.
Doing this will cover your L-cancels IN CASE you don't auto cancel in BOTH hitlag scenarios (hitting a shield or person). & It will also DO NOTHING if you autocancel (optimal). It literally covers your AC, LC, & hitlag problems in one swoop. But you pros already knew that & this is so 2000 & late that I'm sharing. Wrong. You pros were so proud you didn't even read into his problem right. You're welcome. Er, i mean thank Kadano.