Based on what I've seen:
50 xp: 20-40 BR - single cutter, lynx, privateer, pickle. Only encountering low-ranked enemy captains.
67 xp: 20-40 BR - single cutter, lynx, privateer, pickle. Enemy captains have higher ranks, consistently seemed to put up a harder fight.
84 xp: 50-80 BR - single brig, snow, rattlesnake, navy brig, mercury. double cutter, double pickle. Double ship spawns seem to pair higher and lower ranked captains.
101 xp: 70-90 BR - single navy brig, single mercury, double pickle, paired brig or snow with pickle. Haven't re-tested this rank thoroughly for rattlesnakes, but would expect them.
118 xp: 100-140 BR - single Niagara, Cerberus, Renomee. double snow, double brig, paired snow and brig. Paired navy brig or mercury with pickle. Double navy brig, double mercury, paired navy brig with mercury. As above, haven't re-tested for rattlesnakes. I believe I've fought single Surprises at this rank, but I'm not sure.
135 xp: 110-170 BR - single Renomee, Surprise, Frigate. Double navy brig, double mercury, paired navy brig with mercury. Niagara or Cerberus paired with Navy brig or Mercury.
152 xp: 170-250 BR - single Frigate, Belle Poule, Essex, or Trincomalee. Double Niagara, double Cerberus, double Renomee, or paired combinations. Surprise paired with Niagara, Cerberus, or Renomee.
The crazy Flag Captain jump:
169 xp: 500-600 BR: single 3rd-rate, single Bellona, single St. Pavel. Double Constitution, double Ingermanland, paired Constitution and Ingermanland.
186 xp: 550-600 BR: Appears as above, with the exception of the single 3rd-rate spawn, which I have not seen occur.
The crazy Rear-Admiral jump:
200 xp: 900-1100 BR: Cannot be definitive, as usually we do 169-186 fleet search and destroy in preference to solo 200s. Single Santisima Trinidad, single Victory, two Bellonas.
Personally I'd prefer some differentiation between 169s and 186s, along the lines of:
169xp: 470-550 - single 3rd-rate, single Bellona, Constitution or Ingermanland paired with Trincomalee, Essex, Belle Poule, or Frigate.
186xp: 600-850 - single St. Pavel, double Constitution or Ingermanland, 3rd-rate or Bellona paired with Constitution or Ingermanland.
But currently the scaling at that level suffers from the lack of an intermediate third-rate and a genuine second-rate ship.