The rounding is dependent on the stat being calculated. Since some weapons will go to both base and other damage values, there's no one answer per weapon.

If a unit only has an LRM-5, it goes to the LRM special ability and the IF special ability first.

With only 3 damage, it does not qualify for the LRM special ability, so the 3 gets dumped back in to base.

Base damage (at medium and long) is 3/10 = .3. Base damage is always a round up, so this is a 1.

IF special ability is 3/10 = .3. IF damage is always a round normal, so it's an IF0.

So it's not that the LRM-5 is a round up or round normal, it's the stat (base damage or IF in this example) that determines the rounding.

So for the Streak LRM-15, if a unit had only a Streak LRM-15. It does not go in to any special abilities (it doesn't go to LRM or IF).

So if it's just the Streak LRM-15 on the unit..

Base damage is 15/10 = 1.5. Base damage always rounds up, so this is a 2.

The Wendigo C has an Improved Heavy Large Laser, 2 Improved Heavy Medium Lasers, an ER Small Laser and a Streak LRM-15. None of these qualify for any special abilities, so they are all going to base damage only.

At medium range, that is 16 + 2*10 + 5 + 15 (in the order listed above). The total base damage at medium range is 56.

It builds up 41 heat (including movement) and only has 30 heat dissipatoin.

(56 * 30) / (41-4) = 46 (the last step of heat adjustment is taking the 45.405 and round up to nearest whole number)

46/10 = 4.6, which rounds up to 5 (because base damage always rounds up).