That is a good tool.........but.......it only tells the user how many watts it takes to hold the bike to a certain speed down a hill.
It doesn't tell many watts it takes to actually stop the bike on a downhill (e.g. stop sign on bottom of hill). Peak watts for this will be much higher depending on how quick the stop needs to be.
You can use mechanical brakes for that. If you slow down by using regen first, energy un-harvested by regen will be minimal... KE is a function of weight and speed SQUARED.
A 165 lb rider on a 50 lb bike going down a 8 percent grade needs 499 watts braking force to hold speed at 20 mph. That is quite a bit of braking force needed considering the weight of rider/bike and grade of slope. It's quite easy to imagine situations that would commonly exceed that.
Therefore I can see why OEM ebikes with geared hubs don't have locked clutches when the most common battery size for them is 350 Wh to 500 Wh.
It is just so easy to see in many situations for many riders a good amount of energy won't be regenerated because the battery charge rate is the bottleneck.
As far as the mechanical brakes go a person shouldn't have to use those except under extreme circumstances. The Regen system should be able to stop the bike in a reasonable distance without wasting energy. This so a person can ride their bike as the they normally would without compromising Regen.
The fact that in RARE circumstances safe regen might overwhelm battery is the reason for limit on regen battery current in the controller, not to dump the concept altogether. Are are suffering from a particularly bad case of purism - this stuff is nasty, you know
. A combination of regen and mechanical braking works wonderfully by simply using a typical e-bike brake handle, you do not even need a variable regen one. Press lightly - the mechanical brakes don't touch the disk yet, but the motor engages and does the regen, scrubbing speed in efficient manner. For panic/complete stop - just press it hard!
Btw, when you brake with, say, 500w of battery current, actual braking power might actually be close to double that depending on phase current - when it comes to regen braking, even LOSSES provide useful braking force!