More than you think and more than enough to fly a 7 DME arc or an NDB procedure. Not enough to fly direct to a 6 DME center fix using the 1950s equipment of this hypothetical exercise. I'm of course handicapped by my limited in-flight mental arithmetic talents. Which is why I'm asking the question: I'm an avid student, eager to pick up nuggets of wisdom as they appear.
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Have a mental model of where you are in relation to the airfield at all times as your capacity increases this model also includes traffic that you can hear as well. This will help because you will the know when to expect the next vector which can help you plan when you slowdown and when to configure (I know maybe not much a consideration when your doing 130knts but it is when your doing 230-250knts). So from my mental model I would be thinking 300 but would expect it to be tight. The actual point your aiming for is 1-1.5 miles out from the center fix to the north. Then see what the needles are telling me and adjust to get to 7 miles out with 10 degs on the NDB to the runway QDM.
Arith method
15miles out is a quarter of 60 so each degree is quarter of a mile so 40deg to the left currently your 10 miles laterially to the right of the airflield you want another 6 miles ontop of that which is another 24 round it up to 25 to give a bit of space so add that to your 40 you get 65 round. As for wind drift you can start with 5 And adjust. Once you get nearer then adjust keeping in mind you turn in distance which if you haven't been told is move two decimal places on your gound speed and half it to give your distance (same as joining an arc) per 90 degs but you need to adjust that for the wind always with a turning into a tail wind add a bit of extra. So ground speed of 150 knts is 1.5/2 = .75
20 miles out is a third of a mile to a degree
15 miles is a quarter
12 miles is a fith
10 miles is a sixth (or add 10% on to the 12 mile or double the twenty)
So arith method 360-65-5 = 290
So really there isn't that much arith involved.
60/15 = 4degs per mile
40/4 = 10 miles
6*4 = 24
40+25= 65
360-65-5 = 290 I would expect that to have plenty of room.
And because I haven't done a mental sensible check I would have done in the cockpit looking at the instruments I have turned the wrong way and set up for the wrong end and coming in from the north
Its alot easier with your beam bar set up on runway direction and referencing it to the ADF and aslo in the air you have your pilot head on and not running an abstract problem.
Anyway i will leave it as you can see the logic with the arith and also spot where i went wrong.
And for the proper question what I would do is turn 20 to the left and then when I was 5 deg off runway QDM turn for a 30deg intercept ad that will get you onto it at about 6 miles. Your about 10 miles away from the center line and you want to chop 4 miles off from the 11 that you would have if you had continued straight ahead. Which is about 25degs but you don't want a 90deg intercept. So knock 5 off and the wind will help you not get tight.
And I also do this in an aircraft designed by Handle Page which Beagle hates with a passion.