It seems that you do not have a
veset kavua. In this situation,
perishah is halachically required only during the specific days on which you are considered most likely to establish a cycle. Therefore you are correct in observing the three days of
perishah:
yom hachodesh, the
haflagah, and
onah beinonit.
You do not need to observe
perishah on the intervening days. And after the
onot perishah have passed, you do not need to refrain until you get your period. You and your husband are permitted to each other except during the specific
onot perishah.
When it is that close to your period, however, it might be prudent to wipe externally with a
tissue (not after urinating) to see if the bleeding has started. If the tissue is clear, go ahead with relations. If the tissue is stained, you should abstain but you are not yet rendered
niddah.
