How much under? Around 285-300 on a 0.25 is just about perfect.
With a 0.25g BB, the maximum FPS is 313, as it gives the same energy (Joules) as 350FPS with a 0.20g BB.
If you set up your rifle to be at, or only just below, the limit there is always a risk that you will be overpowered on a site chrono when you go to play; chronos are not consistent.
It is better to set the rifle up to be at about 1J (328FPS on a 0.20g BB or 295FPS on a 0.25g BB) and fit, for example, a 60 degree Maple Leaf Macaron hop rubber with one of AK2M4's red nubs. You will see a noticeable improvement in range and consistency, especially if you also fit a better quality barrel.