Data:
Spawn for attacking team is almost always 12s. Defending team usually in range of 16s-22s. Convert original ratio:
Default is 20s attacking 30s defending. Apply that rule to 12s attack and u get 18s defending which seems to be good starting point.
Tweak options:
Attacking team spawn seems to be very stable around 12s. I would recommend only fiddle with defending team spawn if it is feeling unbalanced. Rule of thumb:
Attacking team: 12s
Defending team: 18s (balance options for defending team -2s, +2s, +4s)
Conclusion:
4 possible settings come out of this rule that cover most of the current map settings. This guideline should work for most maps.
12s/16s
12s/18s
12s/20s
12s/22s
Lower reinforcements times for defending team in smaller maps and higher in bigger maps seems to be a pattern as well. Possible due length that attacking team has to cover or it is easier to defend bigger open areas as team can scatter better.
Dynamic map bias:
When dynamic map bias is brought to game (https://github.com/etlegacy/etlegacy/issues/2317) it will be easier to decide which maps need adjustment and effect can be seen in real time.
Server Data:
Some notices:
Frostbite: Mentioned in this post. Setting it to 16s on defense seems logical thing to do and it will share exactly same settings similar to other smaller maps.
Oasis: 15s allies spawn is a mystery?. Make it 12s. Mostly hard map for allies and long distances to cover.
Venice: 16s defending team seems out of pattern for bigger maps and it is usually hard map for allies. 18s is safe option or 20s if you brave enough.
Goldrush: Can be made to 12s/20s just to get it on 12s attack too.
Special delivery: 18s on defending team might be viable option as it is usually easier map for allies
Full server usually makes it easier for defending team.
Another thing to keep in mind is that setting that works in 20vs20 might not be ideal in 10vs10. Since main server is big server and in peak time most players experience it with lots of players so i also recommend taking the games happening with full/almost full server into account if deciding to make any change.
There is ETL 2 server that will never reach this high amount therefore it is also logical that some maps could use different adjustments there