I’m writing a book that requires frequent conversion of currency values between Indian Rs and British pounds. The various websites (not many) seem to offer very different answers. Or maybe it’s my poor calculation. Any resources that you can suggest?
A large part of your problem is likely to be that your period of interest covers 400 years during a time when there were major changes to Indian currency. There were different currency systems, which didn't readily convert and even within a currency system the value of a specified coin depended on who minted it as weights varied. There wasn't a standard Indian rupee until 1835 when the Madras rupee of 180 grains of silver (minted by the British East India Company in Madras) became broadly accepted. Until you get an answer that more closely targets your question you might want to read this old post discussing currency problems in India.