Upgrade combat armory and production time of weapons

Upgrading the combat armory promises increase in production speed by 100% from base vallue each time you upgrade.
http://s7.postimg.org/dftfr1yqz/lvl_5.png
http://s7.postimg.org/4yyitvjfv/screenshot_158.png
http://s7.postimg.org/9yvz1tp2j/screenshot_159.png

Level 1 to level 2 halves production time from 5 min. to 2:30 min. So in the same amount of time I get double the amount of weapons. That's indeed 100% more weapons.
So far so good. I still have faith in my own math skills.

For lvl 3 I would expect production time to be halved again. So 1:15. It is 1:40 though.
http://s7.postimg.org/ux24zwox7/screenshot_160.png

For lvl 4 production time is 1:15.
http://s7.postimg.org/9oogihagb/screenshot_161.png

For lvl 5 production time is 1:00.
http://s7.postimg.org/yj7ycjvaj/screenshot_162.png

So, in other words, the increase in speed is 150 sec. - 50 sec. - 25 sec. - 15 sec.

Can someone explain the math to me? Must have something to do with the 100% from BASE LINE, I just don't see how. Conclusion to me though is that the upgrade to lvl 5 isn't very much worth it. Too little increase in speed for that amount of granite

Jude

lvl 1 5min -> speed 1 / 5min
lvl 2 +100% speed to baseline -> 2 / 5min -> 1 / 2,5min
lvl 3 +200% speed to baseline -> 3 / 5min -> 1 / 1min 40s
lvl 4 +300% speed to baseline -> 4 / 5min -> 1 / 1min 15s
lvl 5 +400% speed to baseline -> 5 / 5min -> 1 / 1min

ok, think i have got it!
taking the base rate of 5 minutes which is 300 seconds:
lvl 2 = 200% faster = twice as fast, so 300/2 = 150 seconds = 2m 30secs
lvl 3 = 300% faster = x3 as fast, so 300/3 = 100 secs = 1m 40
lvl 4 = 400% faster = x4 etc 300/4 = 75 secs = 1m 15
lvl 5 = 500% 300/5 = 60 secs = 1 min



and durin beat me too it XD

Thank you both :)