The M rating of a spring is what it ought to give out in metres per second. Though it's only a rough guide, as there are numerous things that can alter it, such as the efficiency of your gearbox with regard to air seals, as well as your hop unit's air seal and also your barrel length and bore.
Easiest way to work it out is to times the value of the spring by 3.3, as there's more or less 3.3 feet to 1 metre. So an M105 would give you something in the region of 340-50fps, give or take about 20 either way, to account for all the variables.
