It is a bit different...
example:
4 TG
100%+4x20%= 180%
2 TG 2 hydra
(100%+40%)*(100%+40%)=196%
2x TG 1x hydra 1x skel worker
(100%+40%)*100%+20%)*(100%*15%)=193,2%
1xTG 1x hydra 1x skel worker 1x +20% on element
(100%+20%)*(100%+20%)*(100%+20%)*(100%+15%)=198,72%
3x hydra 1x skel worker AND mask of zealotus (+5% on demihuman)
(100%+65%)*(100%+15%)=189,75%