Well you are correct about the too powerful part. It is fine to be too much but not to be too little.
But here is the math for your tank. Start with turn over rates. you need 1.5 times an hour for level 2 and 3-4 times and hour for level 3. So for your tank of 42 gals you need a flow rate of either about 65 gph for level 2 and between 125 and 170 gph for level 3.
Now given that level 2 wants a flow rate through the unit of 20-30 gph and levels 3 wants under 8 gph, the next math determines how many watts the unit one needs should be at the very least. In your case that would be:
Level 2- 65 gph means 2-3 watts and Level 3 is between 125 and 170 gph so a min UV wattage is betweenax rate of 16 and 21 watts. These numbers are calculated by dividing the turnover rate needed by the gph for any given sterilization level.
65/20-30 gph = between 3 and 2 watts- since the smallest usually one can find is 5 watts, this would work. But if you want the max. killing power you are at 125-170/8 gph or less and this works out to 125/8 = 15.6 (16 w) to 170/8 = 21.25 (21 w).
Now in your case your filter will push a max of 1000 lph but that is only if the head is 0 (which it never is) and the filter media is not very clogged or even in it. At a head of 1.5 meters your filter moves 0 water. So lets work with your tank at 40 gals. (likely it is even a bit less allowing for decor etc.), and a flow rate out of your filter more like 700 lph. (about 185 gph).
Your needed turnover rates are:
Level 2- 40 x 1.5 = 60 gph
Level 3- 40 x 3 to 4 = 120 to 160 gph.
Now, to figure the needed UV strengths based the the flow rate and thus dwell time (how long the wtaer is inside the ubit and exposed to the UV light):
Level 2- 60 gph/20-30 gph = 3 - 2w
Level 3- 120 to 160 gph/8 (or less) = 15 - 20 watts.
Now if one increases the flow rate, the wattage needed changes. But in your case you have too large a unit needed for level 2 but OK for level 3. What this means is that you can be OK using your unit to achieve level 2, you are much stronger than that. However, if you wanted to go to level 3, then the flow rate is a bit on the high side. At 18 watts you are in the middle of the desired level 3 range at 160 gph. At your gph you may not quite achieve level.
Your 18 watt unit compensates for the higher flow rate and hence lower dwell time because it is stronger than needed
All of the other stuff in that article gets more complicated. But what it boils down to is this. To have effective UV requires a quality unit in terms of the water distance from the bulb and the type of UV bulb a unit uses. This can all be solved by simply using the brands suggested in the article. The other issue is bulb life and UV unit cleaning. If the quartz sleeve inside gets dirty, light is blocked, if the bulb runs for 6 months, it loses effectiveness and needs to be replaced. At this time you can clean the unit.
It is rare a UV unit needs to be run 24/7 especially once a pond or tank is up and running in a stable fashion. The fewer hours a day or week it needs to be run, the longer it will last. One note, do not run a UV unit during the initial start of cycling especially when seeding a tank with bottled bacteria or squeezing/rinsing out media from an established filter into the new tank. The bacteria take a bit of time to get out of the water and attached to the right places. Running a UV during this period will kill them. Once they attach in a day or two you can turn it on.
Finally, i can ell you I have a Terminator 5 watt unit i got from the site with the article. it was my 2nd choice. I wanted the next model up, but it was not in stock and the next model up after than was way overkill. I mostly have run that 5w unit on a 29 gal. tank with a powerhead rated at 106 gph. The have since upped their min wattage from 5 to 7.
/americanaquariumproducts.com/AquariumUVSterilization.html
