This is missing in the config file.  I can not find it anywhere.

it was removed and replaced with Max Amount Allowed To Automatically Spawn Per Difficulty (0 = infinite, -1 = disabled)

i've updated the doc

We run a very small PvE server, so we only have like 2-3 raid bases at a time on the server.  How can I use this config to be comparable to like Easy bases spawning 40% of the time,  Medium 25%, hard 15%, expert 10% and nightmare 5%?    I didn't want a lot of raid bases on the server

you can't. the previous setting did not do this either, it was a MAX of how many could spawn, not how often they can spawn

it's the same as the new setting except it deals with actual amounts instead of percentages, and it doesn't allow you to spawn more than the set amount when specific difficulties are disabled based on the day of the week. with the prior setting if you set the max for easy at 40% and no other difficulty was enabled then it could spawn 100% of your max events. so if you had 10 max events it could spawn 10 of them instead of 4. the latter will be re-added as an option later on

the setting that you're asking for doesn't exist and would have to added. 

the closest you can get for now is, for example,

set max events to 3 in scheduled or maintained events and then set the amounts for each difficulty to 1 and it will have a 20% chance to spawn 3 different difficulties.

difficulties are randomly picked rather than spawned in a specific order which means you can increase the amount of a specific difficulty in order to reduce how often the other difficulties can spawn. even doing so, this will not exceed to max events set