Yes, as both previous posters said, printf/sprintf. The most flexible format choice is %g, which will swap between scientific notation and fixed decimal depending on precision and behaves most like Perl's default. %f means always use fixed decimal and %e means always use scientific.

In your case, I would probably also round numbers, since I have rarely seen the need for more the 3 significant figures for output outside of scientific computing. So my go to format here would probably "%.3g" for easy digestion, though %.8f is what meets your spec...

printf "%.8f\n", $counts / $total;