I don't want to knock the graph-paper method, but the problem is that once you've got the plants in the ground, they tend to have their own ideas about how big (or not) they want to be. There's only so much you can do about that.
There's also the nature of the plant itself to contend with. Some geraniums will spread and spread, which can be bad for any plants nearby. For example, I once had to fill in a space next to a fully-established geranium 'Patricia', and I decided on a group of lobelia 'Tania'. These are very upright, narrow plants, with dark leaves and shocking pink flowers. I put out my babies, and soon the geranium was snaking around them. Fine, once they get big enough not to be smothered, but until then I had to keep cutting back. Nature does indeed abhor a vacuum, it seems.
It's probably best to plan around the behemoths right from the beginning, because when you first buy your plants, especially if it's by mail order, they're often in those 9 cm pots, and they all look tiny and innocent. It's only after they've put on some growth that you realise that your miscanthus, for example, is beginning to resemble a bamboo grove, and the rubdeckias are taking over the border. Oriental poppies are another one to look out for.
Some plants are just complete takeover merchants - centaurea montana or mountain cornflower is a good example, perhaps because I should never have put it in lush border soil in the first place.
|Centaurea montana, photo by Simon Ross|
On the other end of the scale are the frail darlings that you may love, but can't stand up to any bullying from the others. My iris chrysographes was one of those.
|Iris chrysographes, Creative Commons, by jacki-dee|
Everyone's got their own favourites, who get pampered while the rest have to fend for themselves. Maybe that's why they get so big and brawny; they know in advance that there's no special treatment for them.
In the same way, once you've had the experience of half your plants running amok and smothering the other half once or twice, you're on your way to understanding the science of spacing.