Tip: frame your estimates in terms of intervals with confidence levels, i.e. "90% probability that the answer is within <low end> and <high end>". Try to work out both a 90% and a 50% interval.

I've found interval estimates to be much more useful than point estimates, and they combine very well with Fermi techniques if you keep track of how much rounding you've introduced overall.

In addition, you can compute a Brier score when/if you find out the correct answer, which gives you a target for improvement.