I find with marines, that if you keep the highlights consistent, you can get away with a bit more.
What that means is if everything is done the same, and your highlights aren't razor thin, but at least consistent, it won't look bad.
Might not win golden daemons, but it will make for a very striking army.
However if you are really looking for the clean crisp highlights, consider thinning your paints, investing in good brushes (
GW ones are pretty good actually) and spending some time practicing.
Draw some lines on paper, make them as small as possible, try to get consistency, essentially train your hand and eyes to do what they have to.
And as others have said, paint with a nearby light source to see where you should be highlighting, you can save a lot of work by only highlighting the areas where light would actually catch. No sense highlighting every angle, marines may be superhuman but rarely fight inside a Star