Ann: Quarterly Activities Report and Appendix 4C, page-35

Of the 200k that was chewed by the past losses, there is no CGT discount, so it's effectively dollar for dollar in tax you're paying, only you've already paid it in the past when you solidified those losses.

If there was no past losses to offset then your entire 400k could get the 50% discount leaving you with 200k in CGT.
Having heavy losses banked mean you miss out on the possible discounts.
So the 300k figure is the 200k that went against the losses at no discount, plus the 100k after the discount was applied to the remainder of the profits.