A simple glide ratio value may be provided in the aircraft's Pilot's Operating Handbook (POH). Alternatively, if a Glide Range chart is provided in the POH, you should be able to interpolate a glide ratio from that. Last, if your POH provides information about a glide distance from an altitude at a certain speed, simply convert the distance (run) to the same unit value as the altitude (rise). Once you have done that, divide the run by the rise and you will get your glide ratio at that speed.
As an example, if the POH indicates your aircraft can glide 1.5 NM for every 1000 ft of altitude lost at 85 knots, you will need to do the following.
First, get the units for the rise and run to be the same. In this case, convert the 1.5 nm to ft. 1 nm is equal to 6076.12 ft. The math looks like this:
1.5 nm * 6076.12 ft = 9114.18 ft
Now that both the rise and run are in the same unit value (feet), the ratio can be calculated. You will divide the run by the rise. The math looks like this:
9114.18 ft /1000 ft = 9.11418
You can round 9.11418 down to 9.1. In this example, the glide ratio provided in the POH comes out to be 9.1:1. 9.1 can be entered into the Glide Ratio line within ForeFlight.
Image 1. Enter the best glide speed and best glide ratio on the MORE page under AIRCRAFT > AIRCRAFT PROFILE > GLIDE PERFORMANCE.
Image 2. Alternatively, enter the best glide speed and glide ratio on the MAPS page under SETTINGS > GLIDE SETTINGS.
To add a margin of safety due to real world variables, a more conservative glide ratio of 8 or 8.5 may be utilized.
If you can find no glide ratio information in your POH, then contact the aircraft manufacturer to see if they offer glide ratio information for your aircraft.