Hmmmnnn… You obviously have a GBP denominated account. In order to calculate the info. that you need you’ll need to do currency conversions when trading pairs where the base currency is not GBP e.g. EUR/USD. But I’m sure you already know that hence your post.
Have you tried BabyPips’ own Position Size Calculator??? Maybe it will work by your inputting figures on a trial and error basis to eventually get to the figures you’re looking for??? Just a suggestion. Otherwise a plain old spreadsheet could do the trick. Whichever method or application you use: you’re going to have to know the value of each tick (pip) and that depends on the lot or position size being taken of course.
As for the last part: FORGET IT. You WILL nail your account (and whatever else) to the wall. You CANNOT calculate a position size based on the required margin expressed as a percentage of capital in your account. With leverage (especially the copious amounts of leverage offered on FOREX pairs) this will enable you to open HUGE positions and it will not take much to wipe you out if a few trades go against you. So forget about that idea right now!!! LOL!!!
If you don’t come right with this gimme a shout. I wrote a CFD Position Size Calculator some years ago which I use daily. Maybe it could be modified for your purposes (will just have to restore the source code from a backup and see).
Link to BabyPips’ Position Size Calculator: