One mg increase in a pad weight equals one ml of urine absorbed, says Chris.
Is that correct?
Assuming a urine density the same as water - which cannot be too far off - a milli-litre of urine would weigh a gram, not a milligram. Absorption by the pad fibres doesn't affect the weight, as long as none trickles away down your leg before weighing.
Anyway, no kitchen scale can read milligrams, only grams.