A friend of mine once built an Acorn Atom add-on that allowed him to
measure resistance or capacitance by timing the frequency or period of an
RC oscillator. For high frequencies, it's more accurate to count cycles.
At low frequencies, it's better to time the period.

At some frequency between, the two methods are equally accurate.
He worked out this crossover point, and thus the accuracy was always
guaranteed better than a minimum figure.

You may wish to use this method for your project.