I think that you may be getting to caught up with how much should my lowball offer be, verses, what can I afford and still be able to sleep at night. Ya know what I mean? It concerns me that you would be financing 100% of this purchase with a loan vehicle that may be somewhat risky, at least down the road.
My personal answer to your question is, the correct price to offer is what is suitable for your personal budget. Even if you find a mortgage that equals the rent you pay, don’t forget that you could be buying a depreciating asset, thus if you get stuck in a job layoff or something, you are in a much tighter bind then if you are renting. I am not trying to talk you out of the purchase, I am just saying don’t be concerned with offending this owner or something like that.
If she bought it for 430k I am skeptical that she would sell if for 300k. So what the heck, offer her 250k! Just make sure that whatever you offer, it is a mortgage that you can afford and that you are okay with. Right now the denial factor is still running high among homeowners. Sign a one year lease with her and then hit her again next year. It will be interesting to see what happens then.