The golden rule of estimation is to give a 1-10 confidence level and if your confidence level is below a threshold (7-8 for example) then also define the risks.

The golden rule of estimation is to give a 1-10 confidence level and if your confidence level is below a threshold (7-8 for example) then also define the risks. Finally, use that analysis to give a reasonable upper and lower bound (or be honest and say "I don't know).