The Race Power Calculator (Stryd Membership feature) and the Race Calculations are the best tools to know your race power and times. However, you can estimate your power target based on your CP manually as well:
|Distance (km)||% 10k power|
Your event might be a different distance between 10k and 42.2k, and you can estimate your power for your event with the following formula (does not work for distances less than 10k):
% 10k power = -0.325 * race distance (km) + 103.25
This is a simple linear estimation and is not fully accurate.