First things first, great username.
I am going to answer your questions from the bottom up and base them on a standard waterfall setup. If you are working with something different, let me know.
Frequency capping was originally intended for direct and guaranteed line items to ensure that guaranteed and repetitive creatives were being properly distributed across a site’s readership. Exchange partners do not have this problem. With that said, playing around with frequency caps at the top of your stack can be a good way to grab at the highest possible CPM’s for your best and most unique traffic. I wish I had a perfect number for you but, unfortunately, it differs from publisher to publisher and page to page so experimenting is key. The easiest metric that I would suggest focusing on here is the eCPM and overall yield per partner and ad unit. These metrics will change depending on location within the stack and price floors but testing them against each other will help to get a better understanding of bid distribution. If you do decide to frequency cap at the top, I would suggest implementing secondary tags of the same partner further down the stack to test performance at that mid-quality inventory as well. Be careful with the number of partners that you are working with in this kind of situation to avoid latency and discrepancy issues. Also, if you see that your extra efforts are not producing higher revenue, it might be best to abandon the trial.
At the moment, meridian only breaks out traffic geo by US and all international (other). If you have a large amount of international traffic, specifically if that traffic is concentrated to a specific region, I would suggest breaking out your tags and line items to specifically target those geos. Breaking out your tags by device and geo also provides you with more granular reporting metrics and yield control. The important balance here is between the complexity of your setup and the impact that more and more granular geo-targeting will have on revenue.
Hopefully, my above comments also touch on your original targeting question but, if not, don’t hesitate to reach back out! We would also love to hear how your experiments with frequency capping works for you and the trade-offs that you see, if any, between a more granular targeting approach and an increasingly complicated set up.