I really don't want to argue with you, but it is basic trigonometry. The lights are NOT on top of the fan. However, because the light has a projection that is greater than 90 degrees, it can be affected by the fan. The longer the distance between the ceiling and blades, the further your light needs to be placed.

I'm lazy to draw a diagram for you, but if you study hard enough during your school days you will understand.

In this case i must agree with mrgoodgood on the degree of projection. As we know light projects and travels in straight line and LED lights especially have a stronger projection intensity and specific projection degree. (Eg. eyeball spot light has very narrow projection degree). Normal downlight is not so specific with such projection degree and that's why i said the light is softer or less intensed.

As such if we use these led downlights, we should either move the light further away from the fan blades or use a shorter fan rod so that to avoid the wide projection 'kena' the blades causing shadow flickering (provided the rod is short enough for that certain degree the lights has)... So, longer the rod, easier to 'kena' the light projection.

I know you will say avoid led downlights... but that's still personal choice, as nothing is perfect and each has got its strength and weakness. Although led downlights with more intensed projection but the flickering can be avoided by method suggested. That's one of the benefit of this forum that we learn from people's experience and we find a way to avoid errors...

What if the owner opt for just flatboard plaster ceiling. The fan will definately lower than the lights.

What would be the alternative to achieve lights are lower than the fan?

Err... Pls pardon me but i really don't understand how is your fan in the picture is below the fan level?
Also, what's to do with measuring the fan blades length (instead of distance from blade to light?), and also why is it the ceiling width and length? 

Actually the trigonometry formula is the right thing to calculate. It's not a technical term either. It's what we learnt in secondary math, you had definitely learnt it but you've perhaps forgotten or didn't notice what it calls already.

I've drawn a simple diagram below, with my simple tablet app. Just done it very quickly with free-hand. So, again, pls don't laugh at me!! 



We have got the distance of fan from the ceiling = B
We have the light beam degree already (it varies from different models of lights, eg my light is 112 deg) = C degree
So, we need to calculate what's the minimum A or D to avoid the light beam hitting the fan blades.

To simplyfy my explanation, let's refer to the lighting expert's post on this similar issue and the method of calculation from the following link, post#237.
https://forum.lowyat.net/topic/2938110/+220

Come on bro, your layman method who don't know?? It's only applicable to a bigger room! Especially master room. I am more in a position to tell you this because I experienced before with LED downlight in a small medium room. Did you?? If you refer to my previous post, I already did your layman method which all wiremen do, just common sense right? To distribute it as far as possible and in a uniform way. No need a teacher to teach also a school boy will know! Did you see my few attempts which I move the light further and further away until no more space, until the wall already? The calculation method is here to assist those who haven't install but planning to use LED in a small or medium room, at least they can do something before purchasing and minimize the chances of failure. Of course people can choose to 'try and error' like what I did and of course they need not f**cking worry if their room is big enough! Of course, I need to reinforce again, to use LED or not is a personal choice.

Whether the calculation is working or not, neither me or you can comment, let those who use it and comment. Finally, i don't know whether my post will offense you but I can tell you that I'm very much offended this 'seven morning, eight morning' got to read all these negativity!