How to calculate the best glide speed if there is none on the POH?
According to the FAA, the short answer to your question is approximately &quot;halfway between Vx and Vy.&quot;
While there is a lot of physics and calculus that can give a precise answer to this question, there is another answer that uses &quot;pilot math&quot;. The FAA has a nice publication titled Best Glide Speed and Distance, which explains that there are two different types of &quot;best&quot; glide speeds.

The glide speed that gives you maximum distance covered for a given altitude (Best Glide Distance); and

The glide speed that gives you the maximum time in the air (Best Glide Time).


Per the FAA document, the glide speed that gives you the most distance covered is approximately halfway between Vx and Vy. For example, on a C172 with Vx at 53 and Vy at 73 Max Glide would be about 65. The speed provided in POHs is usually calculated at Max Gross Weight, so your actual Best Glide Distance speed will be a little lower if your weight is less than max gross.
Alternatively, if your goal is to stay in the air as long as possible, then you'll want a speed that minimizes the descent rate. This is typically slower than Best Glide Distance speed, and can be easily determined by pitching for an airspeed that gives you the lowest vertical descent speed on the VSI. There are times when you don't care how far you glide, and instead want to maximize the time in the air in order to troubleshoot. For example, if you are over an area with no suitable landing location (like over the ocean), you'll want to maximize your time to try to restart your engine. Or if you are over your landing location, but want extra time to finish your emergency checklists.