I appears looking at box scores that if a team gets a double minor and kills off all four minutes, they get credit for two power plays killed which makes sense. However, if they receive a five minute major and kill it off they are only credited with one kill. Now this is literally true, but seems to me they should count it otherwise.
I suspect, but can’t find any evidence, that if a team has 2 or more goals scored against it during a major, it’s no worse than allowing just one.
It seems to me they should measure this the same way they measure goals against – using minutes.
PP% = 120*(goals scored during man advantage)/(seconds man-advantage).
PK% = 1-120*(goals allowed while short-handed)/(seconds short-handed).
This would count the two or more goals during a major extra, would give extra credit for killing a major, and penalize a team for giving up goals quickly when short handed.
A number different from 120 could be used for historical comparison. Use the average realized duration of power plays. I’ve made no alteration for two-man short situations.