Hiya!
First, get away from the hard-coded percentages. That's just going to annoy you and your group. Just say "Hey guys, I'm gonna do a bit less 'long speaking'. You guys fill in the details", or something like that. Some nights you may only speak 20% of the time...others, 80%. The amount spoken isn't the value...it's what's being said. Don't get pigeon holed into 'absolutes'.
Second...maybe not play D&D? Not to be flippant or anything, but D&D pretty much requires the DM do a lot of speaking and explaining, describing, questioning, etc. It's just how the game works. There are a lot of "behind the screen" stuff that a competent DM is doing all the time...for me, it's dice rolling, table consulting and note taking.
There are game systems out there that are "Player Driven". Like Dungeon World, for example. When I finally "got it" and the game clicked, I found myself just sitting back and enjoying the Players tell me a story...and describe stuff...and add stuff to the world...and even make up new rules on the spot!
BUT...if you want to keep doing the D&D thing, you need to explain the new expectations and limitations of the Players roles at the table. This relates to my first point, obviously. The "less speaking" is fine, but you need to give the Players some guidelines and responsibility. Not all Players may even WANT this. I mean, they aren't DM'ing, they are there to Play the role of a PC. That means they aren't there to hear you speak less...they are there to not have to do all that DM'ing stuff.
Think of it like showing up at a dinner party with 4 other friends, then your host says "Ok you guys, I got the pots and pans out, the knives and cutting boards, and some ingredients. You guys get in there and start cooking dinner. I'm gonna stand over here and drink some wine and offer general guidance if you want it".

They didn't show up to cook and create dishes...they cam to eat what they want of what you prepared. Some weird-o may even ask for ketchup for his well-done steak! (barbarians!).
Also, as a side note, I find it a bit disturbing that teachers are being told to "speak less"...but that's a different kettle of worms.
^_^
Paul L. ming