Ideally, an NDB or VOR on the airfield will allow you to calculate your displacement from the threshold by just looking at the needle.
The ILS localiser goes alive at 2.5 degress, so as you see the needle hit about 3 deg you know you are close.
If you have a locator, then you will have to apply the 1 in 60 rule. For example, if the localiser intercept is at 12 nm, that is one fifth of 60 and so 2.5 degrees divided by 5 is 0.5 nm. If the locator is at 5nm from the airfield (i.e. 7 nm from you) then divide 60 by 7 (about 8.5). Then multiply the 0.5 nm required displacement by 8.5 to get the NDB bearing that equates to the localiser alive (0.5 x 8.5 = 4.25 degrees).
However, you could also say "the locator is about halfway out so I'll double the bearing to 5 degrees" and you would not be too far out. This can also be used to work out when to turn onto the non-precision F.A.T.
My experiece says that as long as you go rate one turn as soon as the ILS localiser goes alive, then you will be pretty close. And if self positioning, put yourself onto a 45 degree intercept for the earliest possible pick up of localiser (establish early, get settled for a smooth approach).
I am talking big aeroplane techniques by the way - a delayed turn may suit for small craft.