I understand what you’re trying to do here. The thing you want to find out is called the “radius of turn”, so if you look that up you can find some pretty good information.
Here something that will work to line you up VERY close to the centreline of the ILS using autopilot. I just did a quick test.
Test: landing ILS 19L at KSFO
Weather: no winds
First thing you need to do select a waypoint that is positioned directly at the beginning of the Runway you want to land on. Then click on the “Direct to” button in the flight plan page. Now make sure the control bar at the bottom of the screen is able to show “distance to next” and “bearing to next”.
When you’re on the base leg take a note of what the exact runway direction is. For example 19L at KSFO is actually 193 degrees. So if i’m on Left base i’m going to fly 283 degrees.
You want to be positioned about 8.5 nautical miles from the waypoint you selected when you start to turn. This is difficult to guess so instead start flying directly to where the beginning of the red ILS lines begin and it should be ok.
So now you’re ready to turn! At this stage you should have 160kts set on the autopilot and be flying a heading that is perpendicular to the runway making sure that you are flying to the end of the red ILS lines.
- Now you just need to look at your “bearing”. when your bearing is about 9 degrees from your exact runway heading, tune the autopilot to the exact runway heading. and thats it. In the test i did, i waited for the “bearing” to say 202 degrees (202-193=9), then is started to turn and ended up on the ILS perfectly.
Sorry this is very long, I just wanted to explain properly. Obviously if you’re going faster than 160kts or you’re further than 8.5nm away this wont work. If you want to get technical, look up the rate of turn formulae on wikipedia. For your information the “bank angle” is 20 degrees in IF.