When you have the your tanks as parallel as you can get them, run the bolt through the bottom band and snug it down.
Using a set of feeler gauges, check the gap at the bottom and top of your bands to assure that they are not cocked.
If they are cocked, use a wood mallet to make fine adjustments of your band's positioning. If you have no mallet, any piece of scrap wood with a sharp edge will do. The tapping involved here is very gentle. If the bands don't move, loosen your band nut a little bit.
Grab your backplate and use it to check the distance between your bands. Use your lower band to adjust this distance. You want your holes to align up as precisely as you can to make life easy on the dive boat.
When spacing is correct, go through the same feeler gauge procedure as you did with the top bands and then tighten down both sets of bands. Do not over tighten, or you will warp the flats, just tight enough to crush the lock washer. If you have to disassemble your doubles at any time, discard all lock washers and replace with new ones. Stainless Steel lock washers are no good for repeated use.
Looking Good!