Stadium course designers don't just plunk fences down at random distances from one another unless they are many strides apart.
They use the assumption that a cantering/
This means a ONE stride in and out is 24 feet. Six feet in for the landing, 12 feet for the stride, and six feet before the following fence for the takeoff. 6+12+6=24.
For a two stride in and out (and up), simply add 12 feet for the additional stride/s, because the 6 foot landing and 6 foot takeoff remain the same.
So, here's an easy "formula"
Canter bounce=12 feet
1 stride=24 feet
2 stride=36 feet
3 stride =48 feet
4 stride=60 feet
5 stride=72 feet
6 stride=84 feet
And so on, although after many strides, most riders start to "ride off their eye", and jump them as separate fences.
Take a tape measure and mark some of those distances, maybe on your barn floor with bits of duct tape, say in 12 foot increments. Every time you walk down the barn aisle, walk the distances, 1-2-3-one, 1-2,3,two, 1-2-3-three, 1-2-3-four, and so on. Pretty soon YOU will "own" a three foot stride so that you can walk accurate distances, plus, you will know what those distances "mean" in terms of the strides the horse is expected to make between jumps.
Be careful!!---These are NOT THE SAME as "normal" distances. They are SHORTER.
Because gymnastic lines are normally taken from the trot, rather than the canter, and because they are often used to balance and shorten a horse's stride, they tend to be 2 to 3 feet shorter than the 12-24-36-48 you just learned.
A gymnastic bounce might be 9-10 feet. A one stride might be 18-19 feet, a two stride might be 30-33 feet, and so on.