Thanks for your help!
Ah, my long-gone programming and maths days are coming back from the mist (30 years ago!), and so also a sort of understand your comment.
I’ve done some experimenting, and the values entered into the Fire Power level box produce some interesting results. It was quite surprised to see that as values are entered in the level box, so the laser output adjusts immediately. I could sit there watching the laser dot whilst also entering values from 0% through to 2%.
The only times there was a change in brightness was at the following values:
0.1 %
0.2 %
0.3 %
0.4 %
0.8 %
1.5 %
2.0 %
All the intermediate values (i.e. 0.5, 0.6, 0.7, 0.9, 1.0, 1.1, 1.2, 1.3, etc.) made no changes to the laser brightness. I should mention that I often stepped back and forth several times between two values (e.g. 0.4 & 0.5) whilst keeping my eye on the dot and fingers on the keyboard, to make sure there was no change. All changes in brightness were very discernible and consistent.
I fully follow your logic regards the 255 & 1/4%, and never lower than 0.4%, but in practice LightBurn appears to be able to use non-integer values down as low as 0.1% in the Fire laser power box.
Having browsed the forum it would seem that the value entered into the ‘Fire’ power level box should also be the same value when using the Frame function, but it doesn’t seem to be so. Perhaps the two variables used are different types or rounded differently? Pure guessing there!
Either way, as I can get a very faint, but quite visible, dot for focusing purposes using the Fire function at a level of 0.1% it would be lovely for the Frame function to also use the 0.1% power level, as 1% (the minimum it works with) is too bright for the naked eye with a 15W laser.
I guess it would just be nice not to have to don glasses to check the Frame.
Further comments much appreciated 
My controller would appear to be the same as Sainsmart use on their 3018 (atmel 328P chipset?), is non-branded and came from Amazon sold as ‘Vogvigo’.
Again, thanks for your help.