i want to take about 399 images using a shutter speed of 25" set on Canon 7d using an intervalometer. should the interval setting on int be approx 27" and should i turn off preview? does delay on interv mean time to start first exposure?
You might need more than a two-second margin -- on some camera settings, internal noise reduction takes as long as the photo, so it might be 25 or more seconds between shots.
Turn off preview, not so much to save time, but to save power usage. On my camera, I find that the LCD back-light drains the battery more than the exposure itself. A reasonable compromise is to use preview for the first few photos to check exposure and alignment, then shut it (without shaking the camera). Of course, if using an external power supply, leave preview on... the camera should take the next shot even while showing the last.
BTW, don't forget the "rule of 500", if photographing astronomical objects without a clock drive. To prevent noticeable star trails, exposure should less than 500/ f.l.