How do I calculate how much to put on my Oyster card? I will be using the underground approx. 12 times in 4 days zone I-2 plus a return trip by train to Hampton Court. I'm thinking 30 gbp does that sound right?
A Zone 1 & 2 7-day Travelcard costs £30.40, so since you are right around there, I would recommend that, at the very least you break even and if you take the buses or Tube more often, you'll come out ahead.
I don't believe your Oyster card will be worth anything on the trip to/from Hampton Court except to get you to the train station. I think that would be Victoria but I could be wrong. With the 7 day card you can have unlimited bus rides. Even if going short distances it's worth hopping on a bus. Rick's London transportation section in his books has a good summary of the important tourist bus routes. You should definitely consider using the bus options. If you are really obsessed with possibly saving a few pounds, you can go the the London Transport website and price the individual prices for your trips. You'll need to know what time of day you will be traveling because it costs more during the rush hours. The zone 1-2 card won't get you from Heathrow. The tube is cheaper but it takes a lot longer than the regular Paddington train. The express is slightly faster but not much. I've only taken the regular train.
Oyster cards are accepted on South West Trains, so there isn't a problem using it out to Hampton Court on pay-as-you-go as it is in Zone 6. Don't forget to touch out at the platform validators.
It's easy to add more cash to an Oyster card if you need to, and from what you describe I don't think that £30 will be quite enough if you use it for Hampton Court as well. There is a daily price cap which means the total you actually pay will depend on how many of those journeys are on any particular day. Only you can work that out - see the table mentioned below. As Marco says, Hampton Court is in Zone 6, so a peak time single on Oyster is £5.70 and off-peak is £3.50. Trains go from Waterloo (not Victoria). Interestingly, if you bought a conventional Anytime return at the station it would cost you £12.90. As he also says, you must touch out on the Oyster card reader at Hampton Court station (and back in when you return), or you will be charged the daily maximum fare. The Transport for London price comparison chart is here: http://www.tfl.gov.uk/tickets/14416.aspx and the charges for Oyster on national rail (for Hampton Court) are here: http://www.tfl.gov.uk/tickets/14414.aspx