There is NO fixed value of Regen that will work for all downslopes and speeds. So, to be useful in "holding back" the car, the Regen amount must be variable.
Typiccally, there are only two variable controls that are familiar, the go-faster and go-slower. One can split the Regen control between them, with zero to "light" on the go-faster, and "light" to "full" on the go-slower.
Then, "coasting" is not easily available, but might be accomplished by holding the go-faster down a little.
Or, the Regen (a going-slower function) can be entirely controlled with the go-slower. Then, low-drag coasting would be done with the foot off the pedals, or even "barely touching" either pedal. This is not a "keep the same speed forever" mode, the tire and wind drag would gradually slow the car.
On greater-than-gentle slopes, the "little" regen will be insufficient, so one will need to use the go-slower. Occasionally (perhaps not in "flat-land") there will be steeper grades that will require more than the "full" regen, so the mechanical brakes will be needed ... all the way down the "possibly-long" grade, perhaps 1 to 50 miles.
Note: This means a LOT of brake heating! Much MORE than a typical (down-shiftable) car would require. Hopefully, the EV brake designers realize this, and provide brakes that have ZERO heat-fade.
When anticipating stopping, it is best to have one's foot already on the go-slower.
Having the "light" level of Regen adjustable (or switchable) from "light" to "zero" would make the controls satisfy both camps.
This can be a simple Software setting, no extra physical buttons or controls are needed.
During every "anticipated" slowing or stopping sequence, there is the possibility to save energy by not using any, by coasting. Slowing with Regen might only be 50% energy effective. However, when judging when more braking is needed, I want my foot in the safest place, already on the go-slower. In traffic, the cars in front of me can decide to brake suddenly. Also, as soon as I decide that de-acceleration is necessary, I want my car's stop-lights to go on, to give warning to the guy behind me.
Rear-end accidents are the most common here in freeway traffic, and you can stop OK, but giving extra warning to the driver behind ... might save YOUR life, and at least save your LEAF.
See SOC/GID-Meter and CAN-Do Info
2011 LEAF, sold in 2015
2010 Prius, 2014 silver Tesla S
Nissan EVSE, mod to 240/120v 16A
Solar PV: 33 x 225W -> 7 kW max AC
To Sell: X-treme 5000Li EV motorcycle