It's great you are having so much fun with pistons. Unfortunately there is no simple answer to your question about piston volume.
Nothing in model rocketry (with the possible exception of streamer performance) is more complicated than pistons. Piston performance depends on the interplay between a number of important variables.
Here are some of the most important variables:
Motor size and performance (widely variable even for the same motor type)
Rocket weight (see the Alway brothers NARAM R&D report)
Piston tube diameter
Piston tube length
Piston tube weight
Piston tube material (carbon, fiberglass, paper)
Piston cleanliness
Floating head vs. fixed head piston
Piston head shape
Internal vs external ignition wires
Tightness and nature of how motor is held in, and released from, piston
Initial volume in piston prior to motor ignition
The best recent investigations into piston performance, with some very surprising results, were done by Patrick Peterson and the Neutron Fusion team at NARAM 55 and 56.
NARAM 55: [video=youtube;Pdy_DvkZIPU]https://www.youtube.com/watch?v=Pdy_DvkZIPU"]https://www.youtube.com/watch?v=Pdy_DvkZIPU[/video]
NARAM 56:
https://www.nar.org/wp-content/uploads/2014/05/Neutron-Fusion-NARAM-56-RD-FAI-pistons-1.pdf
Patrick's work yielded the fascinating result that piston acceleration is not uniform but quickens and slows and, for a particular model weight and motor combination, actually has almost sinusoidal nodes of peak piston length, sometimes much longer than what we previously thought. Really good work.
So, again, no simple answer to your question but huge fun to play with and explore.
At the most recent world championships in Lviv, Ukraine my teammates Matt and Dr. B each had their own new piston designs. Each will vigorously defend his design as being the best approach. Matt finished first in the world in S5 scale altitude. Dr. B. finished first in the world in S1 altitude.
So I can't tell you whose approach is best, but I can tell you that spending years flying with pistons and constantly tweaking can yield big results.
Steve ("the depth player" whose butt was dragged onto the medal stand by fantastic teammates with better pistons than mine)