1) I set this up in Excel but I'm guessing this work in Google Docs as well. Below are the headings and the formulas. The key ones ae you opening balance and the Return rate, the return rate is anchored and will be the key determinate on the forecast The formula takes the current subtotal * it by the annual return rate (given as 7% in this example) divided by 12 so you get the incremental growth monthly.
The sub total for the next month is the Total Pot from the month before + the new contribution.
2) Once you have setup the first two rows drag it down for as far out as you want to. Just remember this showing the pension forecast on a monthly basis so you might want to add a filter to the headings so you can view it by month (maybe in your birth month).
The are some optional columns I've added, you don't need them but I would highly recommend the third point below:
i) in column B you can add the age you will be in your birth month, handy for a quick reference so you can see how old you will be without having to think about it.
ii) In column E you might want to add in the effect of annual pay rises on your contributions. When I set mine up I used 1.5%, it doesn't make a big difference but it makes it more accurate.
iii) Column I is used for noting the actual valuation of my pension so I can track it's performance against my forecast. That way if you pension is lagging you might be able to make some changes to where you money is being invested (if you pension provider/employer allows it) or up your contributions.