It doesn't make sense to use 20%, because Watson's conversion rate simply won't stay at that. He is early in his career as an opener.
Chanderpaul, 48.98, conversion rate 40%.
Inzaman-ul-Haq, 49.60, conversion rate 54%
Viv Richards, 50.23, conversion rate 53%
Boycott, 47.72, conversion rate 52%
Etc.
You can compare those batsmen who all average around 50, to others who average around 50 and have better conversion rates.
E.g. Hayden, 50.73, conversion rate over 100%.
Hence, different batsmen can still maintain high averages without the same conversion rates.