So sugar lumps are pretty valuable, we know that. But there are two heavenly upgrades--Stevia caelestis and Sugar aging process--that make them ripen sooner, consequently slightly decreasing their value.
I would suggest that the probability of the Sweet! result from Golden and Wrath cookies be increased slightly to account for this, multiplying the threshold by 86400/T, where T is the number of seconds required for a sugar lump to mature, ripen, and drop naturally. For instance, if you have both of the aforementioned upgrades and 600 Grandmas, this would amount to multiplying the threshold by 86400/79200, or 12/11.