It would be great if your ACME API endpoints returned headers that indicate rate limit values, similar to how GitHub does the same thing:
X-RateLimit-Limit: 60 X-RateLimit-Remaining: 56 X-RateLimit-Reset: 1372700873
crt.sh and CTLs are never going to be timely enough for use, and relying on customers to calculate something that you already have values for is also prone to mistakes. Why not just tell us the limits we’re under?
Another bonus here is that we don’t have to centrally manage counters in our distributed systems to see how close we are to a given rate limit – we can just look at our transactions.