Brian, I am sure aquarium gravel comes in different sizes, too, but if yours is really that small, then it would still be fine to use. Average diameter of #1 grit is only 1/16 inch, but your eyes can play tricks on you. In my photo, place your cursor over an average size granule, compare, then bring the cursor down to the ruler and measure.
I used to use #1 grit as a topdressing in special circumstances with very fine seed. But watering from above proved very difficult because grit that fine is not stable and moves around even with the gentlest of surface watering. I can't afford patience for this, as there will always be times when a very quick watering of my materials is required just before I dash off to work. Likewise, bottom watering takes too much time for me, also.
So now I only use #2 grit as the final topdress. Thickness of the grit layer is not an exact science. Most people who employ it are very cryptic when you ask them to be specific. One of the premier Scottish alpine growers, Ian Young, plants his Fritillaria spp.(closely related to Lilium) seed on the soil surface and covers with a centimeter of grit. That would not be too much for hybrid lilies, but I think it is more than you need. I use enough to block enough sun from the soil surface to prevent moss and liverwort growth - about 3/4 of that, or a little over a quarter inch. Except in special situations, my lily seeds are lightly covered with soil before the application of grit.
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Lorn, I reuse my potting soils, too, although more often for potting up something else, rather than adding to the garden on terra firma. Still, my native soil is clay based, and any larger mineral material is a bonus for drainage. I assume you have sand, you lucky man.
Regarding using gravel at the bottom of pots with a separating membrane, you may want to rethink your creation of a perched water table...
