Just use your collected rainwater and don't worry - too much use of the TDS meter will only confuse... Run off from most non-cement based roofs will be fine even if a bit higher TDS than you expect. Measuring the standing water in your trays is almost pointless and is not generally related to water hardness (which is what matters). PM if you want more details, posting results in a fair bit of poorly informed responses. I'm not a water chemist, but am a chemist who's dealt with ultrapure water in the past.