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Book some tunnel time, share the results for the good of the group.
(more of a gap, my bet) -
I think the answer is "it depends" (on frame, wheel and tyre combo) - wind tunnel or some kind of aero field testing is needed to determine the optimum
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About 10mm should be enough to diminish the shear layer drag to a small value without wrecking the flow off the seat tube and onto the wheel too much. As 6pt says, you need tunnel time to find the optimum, but more than a usual tunnel you also need a way of measuring the rear wheel's resistance to rotation while it's in the tunnel, as you're aiming to offset an increase in translational drag with a reduction in rotational drag. On the other hand, if you're just playing on the track then I doubt that there's more than 5W to be found between any wheel position from slammed forwards to hanging off the back.
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48x15 will get you around the 393mm chainstay wheel position mark [bb-center to WP] on a fairly new chainring, chain and cog, do the math from here compared to your frame chainstay/drivetrain setup.
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6pt
danb
gbj_tester
Thuekr
@Brommers
On a Dolan DF4, like this:
(different spec, same frame), do I want the rear wheel as close to the seat post as possible, or more of a gap like in the photo? Common sense says as close as possible, but I know from experience that common sense rarely applies to aerodynamics.