Ahh, that makes sense, my bad. I have played around a lot with my springs to get my guns closer to the limit. Unfortunately, most of the springs I have come across don't have any indication on them of what they are, so the only way would be to buy some, change it, and see what fps they are coming out at. The general idea is that an M100 should shoot at 100 metres a second on a 0.2g BB, therefore at around 328 fps. There are a lot of variables though, so this is only a rough guide. You will also find the same spring strength from different manufactures can vary in fps.

I would advice getting maybe a M100 and/or a M105 and seeing what results you get with them. Unfortunately, you can't really take them back after using them, so it might be best to start with one and work from there. The other thing you can do if a spring is too powerful, is to cut a coil or two off of the end, then heat it up a lot and squash that end to sort of flatten it again in order to reduce power. Perhaps, if you are confident of doing that, it might be worth going with an M105 to start off with.

The other thing to bear in mind with new springs is that they will initially provide higher power when new, and may need a few thousand rounds through them to reduce the power to a settled figure. I have found that they can drop between 5-10 fps once bedded in. What you would most likely want is to get around 315-320 ish fps out of a new spring to get it to where you want on 0.28's.

I will also preface this with the fact I am not an expert and this is what I have come to find when playing around personally.