Be very careful when you are using the technique. 20% of White is only a gauge to use. What you really have to do is to mix the highlight colour to around that composition, and try it. You may have to repeat till you get the correct composition. Also, when the paint is gloss, it is VERY hard to see or notice any difference between the actual colour and the highlight. But by the time u see any, it may be too bright. So the tip to this is perfect lighting
1. Have plenty of light, make sure there is no shadow
2. Use only WHITE light
Also a thing to note here is:- gloss coat tends to make the colour darker, while flat coat will make the colour lighter. Furthermore, when you use this shading technique, the final flat coat may "split" the base colour and the highlight colour. This means that while doing the shading (on gloss surface), it may appear that the shading is just right. But after flat coat, the shade contrast suddenly stands out and ruin the subtle shading which we are trying to hard to get.. this is an example of that occurrence:
http://i25.photobucket.com/albums/c82/EatMeRaw_/DSC02259.jpgSee the dark shade on its legs? It is non-intentional and while on a glossy surface, it DOES NOT appear so bad.. :cry:
Last thing is : Use a 0.2mm airbrush to perform this kind of shading. Use low pressure. short distance and get the correct thinner-to-paint ratio.
---- NEW TRICKS ----
After messing around with a few kits, I have learnt a few new tricks
One of which is known as the reverse shading technique. This technic is basically used for painting red colour.

Also, another interesting note about shading and highlighting. The highlite colour need not or cannot be always white. The main colour should be used to determine the highlight colour.
Red : Red + Yellow
Green : Green + Yellow
Orange : Orange + Yellow
These are only the few I knew...
