Division ratios in baud generators often give slight error, which can be significant at high rates. One way around that, used in the past, is to vary the division ratio of the baud rate generator. Assuming a baud rate of x and available division ratios of x-1, x+1, then use the upper and lower values alternately, or in a count loop, to get a fractional division ratio. Using an interrupt driven timer as generator, it's easy to do that on alternative timer reload interrupts. Can get very high accuracy. Slight jitter on the baud rate, but the uart won't care about that...