If you don't want the expense and maintenance of a meter, for the $20 investment, I think the ColorpHast strips are well worth it. They are inaccurate in a mash (add 0.3), but consistently precise. I'm with Tom - having trouble with the darker beers makes it sound like a pH issue.
Also, I don't think it has anything to do with the fermentation issue, but there's no need to add magnesium beyond what's contributed by the malt. Unless you're brewing with a large proportion of adjuncts, the wort will already contain >100 ppm Mg.