Some 4* troops increase mana speed. For example, Hu Tao, a “slow” hero, needs 12 tiles of mana to use his special. Yet when equipped with a lvl 17 Monastic Battlemages troops, he only needs 11 tiles — because 11*(1+11%) > 12. Yet before lvl 17, he needs 12 tiles still. Thus, lvl 17 is a “breakpoint”.
You can smoothen this out by changing “mana speed increase” to “probability of double mana”.
Under the new rules, Hu Tao + lvl 1 Monastic Battlemages can still get to the special in 11 tiles: eleven tiles + eleven 1%chances of an extra mana point = about eleven percent chance that an 11 tile run will be enough.
For comparison. Under the new rule, Hu Tao + lvl 17 Monastic Battlemages, 11 tiles = eleven mana points + eleven 11% chance of extra point = 1-(0.89)^11 = 72 percent chance of reaching full mana. To get full mana with 10 tiles would be possible - with probability about 22%. To get full mana with 9 tiles is also possible with probability about 6%. Eight tiles would be enough at rate = 6 in a thousand.
In Summary, a chance at a full point is smoother than a fraction of a point (which is worthless until it adds to a full point). Under the new system the number of tiles needed to get full mana would be approximately the same but would be random. There would be very very slight chances of getting full mana with fewer tiles than expected, and a very very slight chance of needing more than expected. As mana troops leveled up, those probabilities would move in your favor.