lets say you're 10 seconds behind the first attack. The formula is Y x 4 or (Y /0.25) where Y is the time difference between you and the time the first march hits. To calculate a 25% speedup, you need to first find the difference between your march timer and the time the first march hits the target.
When you mess up or join late on a time attack or time reinforce you need to use a speedup to catch up with everyone else otherwise if you attack alone your troop loss potential is higher.
HOW TO CALCULATE WHEN TO USE A MARCH SPEED UP? If you are browsing on a computer you can click ctrl + f and search certain keywords to help you find the question you are looking for. This is a War and Order FAQ post with all the questions and answers you might need.
