I would calculate using a spreadsheet, room by room, and then add 10% to the top (because you are almost always missing something!). Remember, it is the cost of replacing what you have, not how much you think your contents are worth now.

Just about everyone is underinsured. And you can face penalties if you have to claim and you've been underinsured. Check your policy for their terms. You are better off paying a little more and upping your limit (and remember to increase it from time to time, to account for price increases and new items you've probably bought) than finding yourself out of pocket when you really need it.

Cost of contents insurance is a bit like car insurance, what excess will you choose, are you itemising things, what about portable cover, cover for motor stuff (ie washing machine, fridge, etc), it goes on and on. Go online and try a few different insurers and don't be afraid to call a few and ask them to match if you have another insurer already for your car. Often your existing insurer will give you a multi-policy discount.