One way is to try to reuse something that's within bootstrap itself (referenced by an existing bootstrap variable, or by using the same class as a bootstrap class). Another way is to establish mathematical relationships where appropriate (e.g. "the margin is twice the padding" or whatever), so that, if you end up changing one thing, you don't need to change all of the others to accomodate. Yet another way, if neither of those other options are appropriate, is to use round numbers, which, in being less exact, can be less brittle.