The standard ante should represent 25% of the small blind. If the chips don't divide like that, you should veer between 20% and 33%, but no less than 20%. In this case, you could choose to go with a standard 25% of small blind ante, but instead they use a 12.5% ante.
Basically, the rule states that you must make ten times the raise size from the total pot when set mining, twenty-times the raise size when calling with suited connectors, and thirty times the raise size from gapped suited-connectors.