Hi, I have an site called FutClub In The Reviews - Websites Section. However I was check a fellow FIFA Ultimate Team Website Called FUTHEAD (http://www.futhead.com/), who have an Tax Calculator For Users, So They Know How Much To Sell Their Players On FUT Market As EA Take 5% Of Sales on The FIFA 13 Ultimate Team Market. So that users know I was wondering if you know how to create something like that for My site (http://futclub.net/)
No idea what the site does, since you didn't give us a link to the calculator, but if you're looking for a net calculation, it's the amount you want / (1 - the percentage charged) or your net/.95 in this case (5% is 0.05). Deducting 5% from that will give you your original amount. How you do it in Javascript or PHP is up to you.
If you look on www.futhead.com you see a navigation bar at the top there is a small search bar there look on the left you see an calculator icon and it pop up's using javascript
What exactly do you want to know - how it pops up or the entire code for the calculator? (They're both on the site, if you search through the source.)