Current rate method
Net asset exposure
BS positions - Use current rate for all A/L positions except common stock (historical rate). Remember that BS positions are static (balances on certain date) so current rate is used.
P/L (IS) positions - Use average rate for all positions.
Remember that revenues/expenses are entries based on periodical (dynamic) basis thus average FX rate should be used, otherwise FX gain/losses will be over and underestimated by translation process.
Temporal method
Net monetary asset exposure
Monetary BS positions - Current rate
Non - monetary positions - historical rate (historical exch. rate means that no exposure to further FX movements)
P/L positions - use average rate only for those related to monetary BS positions, thus SG&A and revenues. For other items related to non monetary BS items use historical rate (COGS, depreciation).
Net income - Here we have additional category and this is mixed rate. It is backed up in fact that this position is based on both, P/L and BS items and monetary and non- monetary, so this is the reason why mixed rate approach is applicable.
Exhange rate gain/losses are booked in P/L under temporal method while are shown in equity (OCI) under current method so financial ratios are impacted by similar consequences as any other P/L vs OCI exposure.
Hope it helps.