cool thing about construct is,
often times, you can translate your thoughts pretty directly to events:
since there is no On Control Released condition (there might be one in the Custom Controls plugin, I don't remember), I'll be using 'left mouse button' instead of "fire" in this example:
we are going to assume that 'power' starts at 175 when you start holding the button:
I want, while the "Fire" control is held down and 'Power' is below 700, to increase the Power in increments of 175 every 200 milliseconds.
Once the "Fire" control is released, the projectile is shot at the speed of 'Power' pixels per second.