I've been thinking about this recently as I'm about to switch up my ratio. Is it ideal to chose a prime number front chain ring and then adjust the back to whatever based on the actual ratio you want? Selecting the largest reasonable prime for a front chain ring allows you to choose a larger rear cog, which will give you more skid patches, so your rear tire will last longer. Is this a good idea? It seems like it's the most flexible option. I'm sure this is old news but, it's just something I've been thinking about as I see lots of 52T and 48T chain rings.