Downwind distance depends on the pattern altitude for the procedure, bearing in mind ICAO recommends about 2nm of level flight to stabilise on the localiser before glidepath intercept, hence if you're using the equivalent of 1,500 ft above aerodrome elevation, GP intercept will be 5nm on a 3 deg GP, so the aircraft will need to be aimed for localiser intercept at not less than 7 nm.