Look at the comparable sales in the area for homes that are similar to the one you want to buy. Make allowances for different locations, condition of the property, number of rooms, etc. and, with the help of your ageny, make what you think is a reasonable offer, but always be prepared for a rejection.
It has been my experience that many of the banks operate under their own guidelines that do not always line up with local market conditions, It would be great if the banks had some logical procedure they all worked with, but they don't.
Just make an offer you think is fair, hope ut will be accepted, but be prepared for rejection.