In XAPP888 (http://www.xilinx.com/support/documentation/application_notes/xapp888_7Series_DynamicRecon.pdf) the Divider settings are set using the High and Low counters, and the value of the counters are doubled to give the over-all divide:
"For example, if you set both High and Low Time to 2, the effective divide value is 4 and the duty cycle is 50%."
How do you set a divide value of 128?
This requires a high and low counter of 64, but the counter values are only 6 bits each resulting in 63 not 64 ("111111" = 63).
It might not be intuitive but the counter values wrap around, so in order to get 64 they should be set to all 0's.
A high counter of "000000" and a low counter of "000000" would give a divide of 128 with a 50/50 duty cycle.