Generally, springs made by the company Laylax are approximated in meters per second. So a Laylax 150 will shoot around 492 to 500 fps depending on MANY things, but these are just approximations.
Springs made by PDI on the other hand, are measured in percents of joules. It's a complicated system that they use because they (PDI) set the base joule of their springs to different values for each gun. For example, a PDI APS2 240% spring means 2.4 x the standard APS2 joule. A PDI VSR10 280% spring means 2.8 x the standard VSR10 joule. Even though this is more confusing than Laylax, PDI upgrade parts are considered to be superior.
Your actual velocity will change depending on the type of piston you have (heavier piston will accelerate slower), the diameter of the inside of your barrel, and how good your compression is.
Which spring are you looking at to use to upgrade?