There are a variety of customs with respect to the Onah Beinonit.
Most authorities hold that the onah beinonit falls on the thirtieth day from the beginning of a woman's last menstrual period. The first day of her period is day one, and the onah beinonit is day thirty. Some authorities maintain that the onah beinonit falls on the thirty-first day from the beginning of a woman's last period. Therefore, some women have the custom to observe both the thirtieth and thirty-first days as onot perishah. Additionally, some women observe onah beinonit for a full 24 hours, from sunset to sunset, of whatever Hebrew date the onah beinonit falls on.
Since this is a matter of custom, we recommend that you or your husband contact your rabbi for his position on this issue. Generally, a hatarat nedarim would be required for such a change, as changing this custom is not automatically permitted.