Grab a tape measure and go to an empty border. Put out markers for just three plants, but do not dig them out yet. Allow space around each marker that matches the eventual mature spread of the plant rather than the actual pot size. This quick exercise helps to show that spacing is easier to understand physically in a border than on a plant tag. That gap you think is too big now is probably what that plant really wants when its stems, leaves and branches are growing.
Start off by measuring the length and depth of your border. Note where your path, edging, wall or existing plants reduce the available area. Next take the mature spread of the plants you want to plant. If a perennial is going to be about 60 cm wide then you must give it a 60 cm diameter space around the middle of the plant instead of a 15 cm pot. Shrubs tend to be a lot bigger when they are fully grown.
Another reason to take the measurement in a border is to get an idea of the growth habit of the plant. Plants with similar mature spread can fit differently in a border. An upright plant leaves lots of open space close to the ground compared to a spreading or rounded plant which fills space quickly. Ground covers spread differently too. That makes a difference if you want to see if plants are meeting each other or shading out other plants or running across a path as the plants grow.
New gardeners often struggle because there is an awkwardness caused by having bare soil showing. There is also the desire to have extra plants to fill the open spaces so your new garden bed does not look incomplete. One option is to cover the open space with mulch until your plantings get going. You might be able to put in a few annuals in some places as well for immediate colour without disturbing the longer term spacing of the shrubs and perennials.
To test the layout you can lay out pots in the locations where you want to plant your plants. Measure between the centre points of the plants and compare it with their mature spread. Walk away from your border and take in the whole picture. Is a taller plant blocking a shorter one from view? Are you planting a border plant too close to the edge so it grows all over the path? Will you still have access to the border when you want to water, weed, prune or deadhead?
Space is not just about equal distances. It is about giving plants room to use and still having a planting scheme that makes sense. Before you dig, look at each of your markers and ask yourself these three questions: How big will this plant be? What else is planted next to it? Can I get to this part of the border later on? If those questions all work together then your spacing is good.