I think the program just calculates the 'discount' automatically - enter the 100% where required and try looking to see if it's only the 90% that's carried forward to the bottom of the foreign pages, or to the grand total of income (that's what Taxcalc does and it pretty much mirrors the HMRC). If they give you the long summary of how they calculate your tax, you'll see where they multiply the amount reported by .9