I've made a few gliders lately, but I think I struggle in finding the appropriate speed to maximize glide ratio.

I make the glider go as slow as it could go (or somewhere very near that point) and then aim the least altitude loss. By that way, the highest point I've achieved is 19.5:1, but I think it's still not enough because real-world gliders can easily do 30:1, 40:1 etc.

If you have experience in that, can you tell me how did you get the right speed for highest glide ratio for your gliders?