Which clipper blades should I use?

[chestnuttyy]

I've got a set of Lister Neon clippers and two sets of blades - A2F (fine) and CA2 (coarse). Ideally I guess I should probably be using a medium, but I don't have any and can't really afford any new ones at the moment. Which would be best to use? My horse is a TB so he's got quite a fine coat but worried using the fine set might leave lines?!

 

[Honey08]

Do a couple of stripes under the belly (not that visible) to see how they clip, then go from there?