I dont use them but if you were using less water I would do as you say pop in less of the tablet - about a fifth of the tablet to each litre of water.. half the tablet to 2 and a half litres of water etc..
If you pop too much of the tablet in it may be too strong and taint the bottles or some could stay on the bottles which wouldnt be too good for LO or if its too weak Im not sure it would do its job properly.
H xx