Mack’s you let the rpm drop down to almost idle before you shift. You can’t speed shift, just take your time. And yes, float shifting is the best way to shift a Mack. You should try a six speed!
I started floating gears today on the down shifting and it seems to work alot better. Seems like 10 mph is the sweet spot to drop from 5th to 4th. Sometimes 800 rpm's work other times it's 1200.... I did look at the year today and its a 2017. And it's 8 speed but it has a splitter making it a 13.