Genetic genealogy is a fairly nascent field of study, and there doesn't seem to be an ironclad cM range guide. The rule of thumb is the larger the shared cM segment, the closer the relationship (because there's less opportunity for a shared segment to "break down" through generations). However, there's a lot of random chance involved, which is why AncestryDNA and others give you a range (i.e. 4th-6th cousins) rather than pinpointing an MRCA.

I'm no DNA expert, but I think 3rd-4th cousins is probably where I would begin my search. For reference, my closest Gedmatch match is an estimated 4.5 generations away with a largest segment of 22.4 cM.

Think of the estimated number of generations to MRCA this way...3.6 means if the calculator ran your numbers 10 times, the results might be 4-3-4-3-4-3-4-3-4-4. In other words, the calculator determined that you are more likely to share a 30.6 cM segment with a 4th cousin than a 3rd cousin. That doesn't take into account 3rd cousins twice removed, and so on.