The only reason to change the height of the fixture is to adjust the uniformity of the light spread out over your grow. So yes a lux meter will do just fine for that.
You don't even need to convert to PAR values. Just make sure that the darkest regions get around 80% of the average light intensity. Doesn't matter in what quantity you measure that light intensity (PPFD, Lux ParW)
To be honest you can simply calculate the ideal hanging height by taking the distance between the rows of light sources. For instance if you have a 4' tent with 4 light sources spread out in a 2x2 matrix, then you'd divide the 4' by 2 to get 2' distance then for 115/120 degree light angle light sources, take 2/3 of that 2' which would be around 16" for a good starting point for the lowest point of the height.
Or explained in another way, the bigger the surface you want to cover ith your, the higher the light will need to hang. IOt needs to spread out further. So you need to estimate how much ground each light source has to cover and then (for bare COBs SMDs) take 2/3's of the longest side of that area for the hanging height.
For a single light per 2'x2' (=24"x24"), you'd need 24" * 2/3 = 16" height to cover that uniformly. When you have one COB per sqft then it would be 12"x12" per COB and then 12" * 2/3 = 8".
In practice it doesn't really matter that much for the light spread if you use quantum boards or COBs, but a QB does throw the light wider than a COB by as much as the size is different. So you can deduct a few inches of height for that.
With led strips the light is spread out extremely well in the direction of the strips. So, with strips you only need 1/2 of the distance between the strips to get uniformly spread light. They can go closer than COBs or quantum boards.