Don't forget that the Interrail pass doesn't completely cover all trains - on some services (e.g. Intercities, High Speed Trains etc) you will be required to pay a supplement.
The ticket will let you travel through all the countries shown on the map of Europe at
http://www.interrailnet.com/2_map_of_europe
As your pricing, whether you have overestimated it depends on what you plan to do:--
You mention you're planning on using private rooms - if you're willing to sleep in dorms, they work out cheaper. I did this last year and paid around €15 - €25 per night depending on where you are (about £12 - £20 per night).
Also, as for food, are you planning to have cheap restaurant meals or buy sandwiches and snacks from convenience stores? If the former, your estimate is probably about right, if the latter, you could probably cut your estimate from £15 to £10 or even £7.50.
Don't forget that you may also want some cash during the day for entry into attractions etc. in the places you're visiting, plus money for the odd souvenir.
Finally, have a good look at where you're going to spend most of your trip - as things are MUCH cheaper in Eastern Europe, this is important in considering in your costs.
Sorry if that's a bit much, but I hope that gives you some idea of what to think about!
Going on holiday, interrailing, broadening one's horizons and weekends in Skegness
Useful Resources:
Interrail Price??
