You can plug an 'automatic' flash with its own sensor into the jack on the Polaroid front shutter housing. If you take your finger off the shutter button right away after the shutter opens and the flash fires the shutter will close. That is your X-synch speed
Here is a breakdown on the apertures available to the 250 (yours may be similar):
(there are two adjustments that change the aperture, INDOOR/OUTDOOR and the film speed)
Focal length: 114mm
Format diagonal: 120mm
Optimum aperture for format: f20
Wide Open: F 8.4 (measured. Specs are F 8.8 per the manual)
300 - Indoors: F 12.6
75 - Outdoors: F 19
150 - Outdoors: F 25.3
300- Outdoors: F 39
3000 - Outdoors: F 50
I set my flash for f22 and the camera for either f19 or f25 (based on the above table) , depending on if the pictures are a little light or a little dark.