Rules for gliding?

[NRSASD]

I'm building a homebrewed monster that can glide. I was wondering if there were any rules outlining how gliding has worked for other creatures to use as a base line? I'm specifically looking for a ratio of height to glide distance, and a per round movement. 5E rules would be preferred, but any rules work in a pinch. As always, thanks for any and all help in advance!

[BobZan]

Give him a "Gliding speed" based on a similar creature fly speed (50-75% of fly speed)

[Unoriginal]

Just give it a speed you deem fitting (ex: 30ft a round), add the restriction it has to move down, and otherwise treat it as flying.

A creature that can reliably glide should be able to contriol its direction and angle of descand.

[Coffee_Dragon]

add the restriction it has to move down

All my characters have had this particular ability

[JackPhoenix]

Look at Wingwear from PotA. It's a magic item with limited charges, but allows moving at 30' speed at the cost of 5' drop.