PvE and PvP answer different questions
PvE gives you repeatable targets and is useful for observing how a lineup performs against known content. PvP adds another player's heroes, troops and account state, so one result is much harder to generalise.
Compare like with like
If you are testing a change, keep as much of the situation constant as possible. Record the battle type, your power before and after, wounded units and the lineup used. That makes later comparisons more useful without pretending the numbers reveal a hidden combat formula.
Use hero and formation context
Pair battle observations with the hero guide and formation guide. A raw power number is much easier to understand when you also know which roles and lineup were involved.