This has been discussed before, try searching the archives with something like "AccountingMax vs RelayBandwidthRate".

I think teor gave a very good answer here: https://lists.torproject.org/pipermail/tor-relays/2015-May/006956.html

The technique you'll want to use really depends on whether you want a faster relay for part of the month, or a stable (but slower) relay throughout the month. Teor goes into more detail in that post.