Here is the formula for calculating your ETA.

Every sector is 25 Miles²

Open your E6B to get exact info or just go by the red indicated line off AIS.
Once you have True Air Speed divide by 60 (minutes)
Once you have that number your going to take 25 and divide it by that result number.

EX ( True 250 ) 250/60 = 4.17

      25/4.17 = 6.00 mins

    to go 2 sectors will take approx 10 mins

don't forget your true speed increases with height.

Level flight doesn't hurt either.

This next section is for damaged engines.

Here's a simple math gauge for just about any fighter in the game that will help you glide home. Here's how:

Just about every fighter in the game can glide at 1000 ft per min descent at approx 120 mph with the prop feathered. You must have feathered prop prior to the engine seizing. ( to feather prop +/- key keypad)
Translated that means for every 2 miles you travel over the ground you will go down 1000 ft.
Right? 60 mph = 1 mile per minute. 120 mph = 2 miles per min. We've established the 1000 ft down.

Now, you also have a pretty good idea of your distance to home. Why? Each square (sector) on the map = 25X25 miles.
So you hit esc look at the map and see you're about 3/4 of a sector away from your home field. If 1/2 way is 12.5 miles then you can figure approx 19 miles (always go conservative) is how far to get home.

 Basically once you have an engine hit, start your ascent home. If its coolant, you may be able to shut your engine down and glide, then refire the engine to gain more alt, don't count on it though, especially if your in the mountains. A ditch is better then a death.