A bonus issue ex-rights reference price equals the prior close divided by (1 + bonus ratio). At 1:8 the quoted price drops to one ninth, but share count rises ninefold, so account value is unchanged.
The theoretical ex-rights drop in a Korean shareholder-allocation rights offering equals allocation ratio × discount rate ÷ (1 + allocation ratio). Samsung Biologics' terms — 0.039 and 15% — produce 0.57%.
The ex-rights base price for a bonus issue is the last cum-rights close divided by (1 + allotment ratio). Here is why a 0.3-share ratio drops the base price 23.1%, with the decline rate calculated for every common ratio.