The idea to "spend dirty fiat first" is flawed. Everyone should be regularly spending portion of their bitcoin.

Why?

A)

If you think bitcoin will become money at some point in the future, then you will have to start spending it some day anyway. Why not start today? You can always "spend&replace" (spend bitcoin and right away buy the same amount of bitcoin for fiat).

The benefit is that this action makes bitcoin economy larger and when bitcoin economy gets larger, the price of bitcoin grows, more people are going to be interested in accepting it, more people will get used to it sooner.

B)

If your target is to be 70% in bitcoin, just exchange that amount of fiat for bitcoin and you are done. Why would want to waste your time holding more fiat instead? Just go to your goal directly and you don't have to worry about anything else.

If you want to automate things in future, then there are options to adjust your income to be split in the 70:30 (btc:fiat) ratio too.