Choose the target deliberately
Start from the question you want answered about a rival or official. Select the relevant townsperson and inspect the investigation options. The diary describes the trade-off qualitatively, so there is no verified best spending tier here.
Record the cost and result
Capture the chosen option, quoted cost, required time and eventual evidence. Keep failed investigations too if you are testing a mechanic. A single outcome cannot establish the likelihood of finding evidence.
Decide the next action separately
Finding evidence is not the same as winning a trial. Review whether it supports charges, blackmail or a sale, and check the Lawbook before another action. The evidence tracker stores notes locally; it does not certify that a case will succeed.