The dose is only half according to Louise newson, the manufacturer thinks the dose should be the same regardless of the delivery method. Perhaps you could try 200mg vaginally first? I haven't heard of anyone on 300mg. Are you still peri as you're on cyclical hrt? Perhaps your own cycle has got out of step with the hrt one (my body is always stronger). Have you been investigated in case you have a polyp or fibroid that's causing it?