Tech level is determined by 1d6, with tons of modifiers based on all other characteristics of the world. In short, good startports, small world size, unusual or thin atmospheres, very low or very high hydrographic percentages and low and high population extremes give a bonus to TL.
The Mongoose Traveller rules also impose a lower limit on TL based on atmospheric type. If the tech level is any lower, the locals are unable to build or maintain the machinery which enables their survival. Their TL list is a little odd, though.
The test run of 10000 worlds gave me this distribution:
The exact numbers at the high end:
197 C 108 D 56 E 35 F 9 G 2 H
The two -H worlds are:
Temperate 40803 A310AC9-H Hi Ht In Na Special 47003 AADAAA9-H Fl Hi Ht Wa
Both would be interesting worlds to detail.
Anyway, the distribution works for me, since I wanted to keep the Tech Level to 13 (or as low as possible). The TL0 spike may be something to keep an eye out for, and of course we’ll have to see how this looks with actual sectors… Time for another trial run.