Kippy,
JTAC will give you a best guess coordinates from their position. But it wont be directly on top of target.
You could visually verify the target using the TPG. Using the different sub modes on the TPG (WHOT and BHOT) you will be able to see through some cloud but not all of it.
If you wanted to fire through the clouds without seeing the target than a CBU would be best to cluster bomb the area the target was spotted.
Also if you setup JTAC to ask for GBU-38 and the A-10's don't have onboard, JTAC will shutdown and say 'No further tasking available'. I always set weapon to Auto to avoid this.
Or you can scroll over the bunker in Mission Editor to get the most accurate coordinates. Then you could set a Msg to be displayed in game with the coordinates for that target. You will have to enter coordinates into your CDU manually.
Hope this helps.