Calculating resistance for 50% duty cycle
The 'ideal' solution is to make the circuit completely symmetrical, by using a 360ohm and a LED in the right hand side. Instead of a LED, a string of several silicon diodes could be used to approximate the voltage drop.

Using a larger resistor than 360 in that position would give 50% duty cycle, perhaps in the ratio of vsup to (vsup-vLED), so maybe around 470ohms. Try it and see. Voltage and temperature variations will not be compensated, and will cause a drift away from 50%. The two LED solution stays symmetrical regardless of supply voltage and temperature.

Using two LEDs per circuit is not as profligate as it sounds, as with 60 LEDs on the ceiling flashing nominally randomly, I doubt you could spot without meticulous study if any pair of LEDs was flashing as an on/off pair, especially if they were some way apart. It would therefore save you half of your transistors. Mounting each led on (say) a foot of wire would allow you to get them two feet apart. 

On reflection, I agree with autistic's comment that 12v supply is too high. When a TR turns on, its collector takes the base of the opposite transistor down through the 10uF to -(nearly)12v, which exceeds the reverse base-emitter breakdown voltage of most transistors, which is almost always in the 5 or 6v region. Some transistor types will go bang at once, some will survive, and Sod's Law says most will gradually degrade to fail after you have glued all the modules to your ceiling. The easiest way is to reduce the supply to 6v or less, though you can play games with diodes and/or resistors to limit the reverse base voltage or breakdown current that flows, complicating an already fairly complicated circuit. 

Does each have its own battery, or is there a supply wire going to each, in which case there are wires all over the place anyway?

It's a pity when one has a large stock of an enticing but inefficient to use component, like those transistors. A much better way to do this would be to use a 74HC14 hex schmidt package as the centre of 6 flashing LEDs from its 6 independent relaxation oscillators. I've made that mistake so many times that I'm starting to notice it in my old age!