Well for a start look at what similar cars are worth on the trader etc then compare them to yours regards mileage etc, plus you must have a figure in ya head what you think your car is worth . Also you have to ask ya self, if I put my car round a lamppost how much would I be happy with of the insurance, like I said you must have a figure in ya head.
Be aware the higher ya " agreed " valuation is the higher your premium will be.For example, I insured my 1.3 Fiesta fully comp with NO yrs no claims at 20 yrs old for £380 simply because I put the value of the car at £100. I mean the policy was only set up to enable me to drive my Dads Cossie 3rd party so it didnt matter,


PS I know I didnt realy answer your question mate but just some points to think about, hope I helped


Also if you put the valuation to high say for instance 7 grand. and you then do 4 grands worth of damage, now that is a lot of damage to a little cabbie, near right of probably, now do you realy want that car back cause if its between paying 4grand to get it fixed or paying you 7 grand to right it of then matey you will be getting ya car back fixed up, another point to consider, so dont be greedy