How much water to ad to black tank when starting out.?

I have a home dump station, so I clean and flush the tank when I get home, drain it completely. Don't add any water or chemicals before we leave. After a days use, I occasionally add some chemicals. You quickly add enough water once you start using the toilet. And we often dry camp for a week or more so I don't want any initial water in the black or grey tanks.

I 9 years we have camped probably close to 600+ days, and I've NEVER had an issue with black tank clogging.