Mike go and look at this thread down in the 'Paint Shop' section of the forum...
http://www.sim-outhouse.com/sohforums/showthread.php?t=41042
In one post I added a zip file that contains a larger version of the grid shown in the thumbnail.
Grab that file, unpack it, resize it if you are working at 1024 x 1024 (it's currently 2048 x 2048) but keep the original so you can use it again later if need be and then place it onto a new layer at the very top of your layer stack. If you don't know how to copy layers from one PS file to another shout and I will explain that bit.
Once you have it in place in your layer stack for the paint then save as a .psd, flatten the image then flip and save as a DDS and when you look at the paint in FSX everything will be covered in numbered squares.
All you have to do now is find the parts on the aircraft that are giving you problems and see what colours/numbers are on them and then open up the .psd file again and find the same colours/numbers on the sheet and that will be where the parts are on the texture sheet. Once you have found the area turn off the grid layer by left clicking on the eyeball in the layer palette and you should be able to see what you need to paint on the main texture.
For small parts it is much easier if you reduce the size of the grid down to 512 x 512 or even 256 x 256 and then slide it around until you have it in the right area. Unfortunately trial and error and a bit of intuitive guesswork are the only things that are going to help you find the correct location.
Another method used to identify parts is to start a new layer at the top and then using a fairly fat paint brush start drawing squiggly lines on the various parts making sure you change the colour of the brush for each part. Try to use vivid and contrasting colours for the area you paint in so the lines are easily seen. When you save the file and open FSX the aircraft will be covered in the coloured lines and all you have to do then is find the part you want to paint and see what colour line is on it.
OR
Use the rectangular selection tool to draw a box on a new layer over a suspect area then fill with a bright colour using the 'paint bucket' tool. Rinse and repeat as necessary.
There are lots of ways of finding the tricky parts on a texture sheet.
