Donating one R-Gacha normally give waiting time of 50 min (90%+ chance) but you can get longer waiting times if you lucky.
I tried to get waiting time from 1 R-Gacha of:
a) 0.50
b) 1:00
c) 1:30
d) 2:00
e) 2:30
f) 3:00

Players newer get shorter waiting time that 50 min with 1 R-Gacha.

Donating one candy normally give 5 min waiting time but you can be lucky and get fx 1:30 waiting time - Reward in this rare case would be like donating 3x R-Gacha. Chance of get-ting 1:30 waiting time with just 1 candy is probably is less than 1%. A super luck person got a limed R-Girl from just 1 candy. In theory I think you could even get limed SR-Girl with just one candy but chance would be like 1 in 1000000 or less ...

The more you donate the longer waiting time will be guarantied.

The longer waiting time you get the better basket the final reward will be drawn from.

I think this Work as intended.

A players who donate en item worth 550 points and a player who donate an item worth 650 points both are garantied 0:50 waiting time.
Just a theory: Maybe the person who donated 650 gets bigger chance of a longer waiting time = better reward than the person who donated 550 points.