I'd say that 1p per point is a reasonable baseline, but the avios are yours. 2p is frequently achieveable. I wouldn't vary the formula based on location though.

If you think you can get more value per point then by all means increase that number; if you have so many avios you never get the chance to spend them, drop to 0.0067 (but no lower because that's the hotel/car/cash reduction on BA flights value).

JFK - CLT is 542 miles but 7,500 avios each way - so 15k miles return or £150 GBP on that formula, plus a small amount for taxes, fees and charges. In that case, it would translate to a break-even point of £150 - but if you're saving them for a big trip where you can get more value per point, use the value for the next redemption...