Testing runoff the way you suggest isnt practical in my case. I run AA aero, which causes two major issues. HPA, if done properly, would also have these issues.
Early in a grow as much as 98% of the nutes I spray into my root chamber never touch a root. Even late in a grow I have very little runoff of water that has ever touched a root. The system is very efficient as far as water hitting a root and NOT running off. Almost all the water that hits a root is absorbed when my system is set up properly, so there is little to no root interaction with the runoff that would change TDS or PH. On top of that, probably 25%-50% of the nute water that does collect evaporates before it collects in my drain basin - which skews TDS way up and PH numbers fluctuate drastically towards the hi side as well.
As a result, its only practical for me to monitor the rez. I dont even try to test the runoff any more.