Topping off can be tricky when you rack between different caboys (especially Better Bottle and glass), because the volumes are a little different. After a few kits/rackings, you get a feel for which carboys are slightly larger and which ones are smaller (as long as you keep the total number of carboys under 15-20 or so!).
That's one more reason I prefer to use glass marbles/beads to top off/displace air at the top of the carboy - with enough marbles, you can accommodatethe different carboys and the different kits. Some kits have a little more sediment/volume early on, so you might start off in your largest carboy and rack down to the slightly smaller size as more sediment falls out and isleft behind. You still end up with 29-31 bottles, but early on, the total volume is more like 32-33 bottles (~6.5 gallons). Another 'trick' that I use that is not really widely accepted (I think) is to hold out some of the water from the start on the lower end kits, because they tend to finish a little 'thinner' than your higher-end kits. So, instead of adding exactly enough water to 7 liters of concentrate to get to the 23 liter mark on the fermenter, I hold out some of the water at the outset so it's about 22 liters or a little more. It usually improves the lower end wines a little bit, but also makes for trickier racking from the outset, because the kit never fills my smallest 6 gallon carboy by itself.