I'm guessing 240 or 250 would be the floor for Space Age. There's not really a lot of wiggle room right now, even if they finally get around to adding temple, alliance gate and (gulp) coalition/embassy upgrades in CWA it seems that would put the max at 320 at most. As it stands, it takes roughly 4 or 5 upgrades to gain a level for me being at 253 CWA so 250ish seems reasonable.