The extra respawn time cannot be set by the scene maker as of now by the way, unless stronghold flags are present according to Fips. Just in case that matters. As for the formula, I had 15-20s in mind as the lowest and 30s at 60 players, so to me that formula sounds good for 30 to 60. I think you should have a low cap though, otherwise at really low population attackers will experience defenders respawning way before they can even get close to the flag.
With 20 players defenders respawn in only 10s, that is too little if you ask me as most flags are at least a 30s walk for attackers even when they opened the quickest route. Ofcourse the defenders rarely spawn next to the flag either, but usually not that far away. That would overcorrect the problem and fuck attackers over at low pop.