It looks the code from https://w.wiki/3RTX doesn't allow an arbitrary percentage yet. The code requires amount of welcomesurvey-enabled newcomers to be specified as 0-n, where n is a natural number smaller than 10. The code then generates a random natural number m smaller than 10, and checks if m <= n. If it is, it shows welcomesurvey, if m > n, no welcomesurvey is shown.
That means as-of now, the percentage of WS-enabled newcomers needs to be divisable by 10 (in another words, we can deploy WS to 0% of users, 10% of users or 40% of users, but not to 45% or 2%, as neither 45 nor 2 can be divided by 10).
Change https://w.wiki/3RTX to use a similar system to wgGEHomepageNewAccountEnablePercentage – specify a percentage (ie. n <= 100) instead of a range, which will allow us to be more flexible.
Filling this as a blocker to T281896: Deploy Growth features on English Wikipedia, as the requirements of T281896 cannot be met without this task completed.