Interest help

matd

I've got £850 (purchases) on a Egg card that I've just BT'd across to my Barclaycard as the purchase rate of 0% was about to end. The BT was fees free and the rate is 6.9% life of balance. If I shift that back to the Egg card, I have a further 3 months left to run of their balance transfer period (purchases 0% was 6 months, just ended 1st Sept, BT was 0% for 9 months, ends Dec 1st).
The Egg card will give me a further 3 months interest free before I move it again, but will charge me a 2.5% fee of the BT (£21.25).

Will it cost less in interest for me to leave the £850 on the Barclaycard for 3 months versus transferring it interest free to the Egg but with a 2.5% fee?
How do I work out the interest on a monthly basis given an APR of 6.9%?

The other option is to apply for a card with another 0% period 6-12 months. My last application was the Egg card 6 months ago, then an application for a Halifax card about 2 years before that (which I didn't take up as the one I got wasn't the one I applied for), then an application about 21 months ago for the Barclaycard. Are these applications spread out enough (6 months since last) not to damage my credit rating if I apply for another card?

nomoneytoday

The charge will be £21.25 (2.5% of 850)

3 months interest on 850 is about £15, so I would leave it where it is..
(its actually slightly less as you will pay off 3 months worth of payments)

matd

Is there an equation (or Excel formula) to work this out for other amounts? Would be a useful thing to have to forward plan finances.