Robn1 is correct, yours is closer to 1/144 scale. At that scale you shouldn't be able to discern any pattern in the tiles because of the tight gap and "step" tolerances.
If you're trying to replicate a "new" shuttle (i.e., one that wouldn't show the effects of re-entry), mask the area and paint it flat black. You're done.
If you're trying to replicate a "used" shuttle, take a look at
this photo. You'll notice the only discernable pattern is around the three landing gear doors and the two square umbilical hatches just forward of the body flap (that somewhat flat section under the main engines). The rest is a random selection of new tiles, semi-new tiles, and scorching. Unfortunately, none of the currently available 1/144 scale decals (and there are very few) replicate this accurately.
When I get around to building my Revell 1/144 scale shuttle in the not-too-distant future (hopefully) I'm planning to mask and paint flat black as suggested above, drybrush using various shades of gray to replicate the scorching effects, then pick out the new and semi-new tiles by hand with a small brush. Time consuming, to be sure, but if you want it done right you have to do it yourself.