What you have described is one way to use WF. Remember that their are various settings that you can change such as number of repetitions and ratio of IS to OS so you might want to try other tests as well as the 70/30 x 5 test. Setting a dummy variable value to test from 1 to 1 or just testing one of your strategies variables with a range x to x  and then testing various combinations of %/% x repetitions can give you a good idea of overall robustness.

The 50% level for WF efficiency is also not set in stone. 50% basically means that your strategy was half as good in the OOS periods as it was in the IS period. You might decide that this is not robust enough and strive for greater than 90%. On the other side of the coin a WF efficiency of 150% indicates that your strategy performed a lot worse in the 70% IS periods than it did in the 30% OOS periods which might be of concern.

If you have a strategy that seems to be quite robust in various WF tests and seems to come up with an optimised variable setting within a close range for all repetitions then you will want to use the last chosen variable if you go live as that is the one that has performed the best most recently. Then you will want to re-WF optimise occasionally to decide if you need to tweak the value occasionally.

There is no fixed in stone right or wrong way to use WF and no set of rules as to how it should be used. It is simply a tool that allows us to analyse our strategies and get a ‘feeling’ for how robust they are.